[Mark Nord]

Analogus,

obvious difference between your definition and the Sony/kinoma one is their use of
variableUpdate="VAR_UPDATE_FONTSIZE"

I would digg into de-fsked kbook.xs to learn more.

For your info please find the de-fsked sourcecode of this.pageSizeOverlayModel in the spoiler.
(No guaranty that there is something useful)

Spoiler:

	this.pageSizeOverlayModel = new this.pageOverlayModel
	this.pageSizeOverlayModel.VAR_MAX_FONTSCALE = this.__xs__number.parse('0');
	this.pageSizeOverlayModel.VAR_MIN_FONTSCALE = this.__xs__number.parse('0');
	this.pageSizeOverlayModel.VAR_RADIO_FONTSIZE = this.__xs__number.parse('0');
	this.pageSizeOverlayModel.VAR_ENABLE_ZOOM = this.__xs__boolean.parse('true');
	this.pageSizeOverlayModel.VAR_ENABLE_STYLE = this.__xs__boolean.parse('true');
	this.pageSizeOverlayModel.VAR_UPDATE_FONTSIZE = this.__xs__boolean.parse('false');
	this.pageSizeOverlayModel.zoomModel = this.pageZoomOverlayModel;
	this.pageSizeOverlayModel.selectStyle = this.pageSelectStyleOverlayModel;
	this.pageSizeOverlayModel.commonMinScale = this.__xs__number.parse('0');
	this.pageSizeOverlayModel.commonMaxScale = this.__xs__number.parse('3');
	this.pageSizeOverlayModel.SHOW = this.__xs__boolean.parse('false');
	this.pageSizeOverlayModel.initialized = function () {
		<global>.pageOverlayModel.initialized.call(this);
		this.SetSub('SIZE', this);
		this.commonMinScale = <global>.parseInt(<global>.System.applyEnvironment('[kFskDocumentViewerMinTextScale]'));
		this.commonMaxScale = <global>.parseInt(<global>.System.applyEnvironment('[kFskDocumentViewerMaxTextScale]'));
	};
	this.pageSizeOverlayModel.closeCurrentOverlay = function () {
		this.zoomModel.closeCurrentOverlay();
		if (this.SHOW == true) {
			this.setVariable('SHOW', false);
			this.targetModel.setVariable('OVERLAY', false);
		}
	};
	this.pageSizeOverlayModel.openCurrentOverlay = function () {
		this.closeCurrentOverlay();
		this.initSizeMenu();
		<global>.Fskin.overlayTool.setShowHide(this);
		this.targetModel.setVariable('OVERLAY', true);
		this.setVariable('SHOW', true);
	};
	this.pageSizeOverlayModel.UpdateModel = function () {
		this.initModel();
	};
	this.pageSizeOverlayModel.onSize = function () {
		<global>.kbook.model.dictionaryMode.clearPopup();
		if (this.zoomModel.SHOW) {
			this.zoomModel.closeCurrentOverlay();
		}
		else {
			if (this.selectStyle.SHOW) {
				this.selectStyle.doCloseSelectStyle();
				this.openCurrentOverlay();
			}
			else {
				if (this.SHOW) {
					this.closeCurrentOverlay();
				}
				else {
					this.openCurrentOverlay();
				}
			}
		}
	};
	this.pageSizeOverlayModel.onInstantZoom = function (zoomPlus) {
		if (this.zoomModel.SHOW) {
			if (zoomPlus) {
				if (!this.zoomModel.checkZoomLock()) {
					this.zoomModel.zoomRateChanged(10);
				}
				else {
					this.targetModel.doSomething('doBlink');
				}
			}
			else {
				if (!this.zoomModel.checkZoomLock()) {
					this.zoomModel.zoomRateChanged(-10);
				}
				else {
					this.targetModel.doSomething('doBlink');
				}
			}
		}
		else {
			if (zoomPlus) {
				this.targetModel.overlayModel.onZoomUp(10);
			}
			else {
				this.targetModel.overlayModel.onZoomUp(-10);
			}
		}
	};
	this.pageSizeOverlayModel.changeFontSize = function () {
		var scale;
		if (this.SHOW == true) {
			scale = <global>.parseInt(this.VAR_RADIO_FONTSIZE);
			scale = scale + this.commonMinScale;
			if (this.isDictionaryShown()) {
				this.setDictionaryScale(scale);
			}
			else {
				this.targetModel.doSomething('sizeTo', scale);
			}
		}
	};
	this.pageSizeOverlayModel.goZoomMode = function () {
		if (this.SHOW == true) {
			this.closeCurrentOverlay();
			this.zoomModel.onZoom();
		}
	};
	this.pageSizeOverlayModel.doSelectStyle = function () {
		if (this.SHOW == true) {
			this.selectStyle.openSelectStyle();
			this.closeCurrentOverlay();
		}
	};
	this.pageSizeOverlayModel.initSizeMenu = function () {
		var model, scale, minScale, maxScale, target;
		model = <global>.kbook.model;
		if (model) {
			switch (model.STATE) {
			case 'PAGE':
			case 'DICTIONARY':
				target = this.container;
				if (this.isDictionaryShown()) {
					scale = <global>.kbook.model.dictionaryCurrentScale;
					maxScale = this.commonMaxScale;
					minScale = this.commonMinScale;
					target.sandbox.initBtnShowHide(true);
					this.setVariable('VAR_ENABLE_ZOOM', false);
					this.setVariable('VAR_ENABLE_STYLE', false);
				}
				else {
					scale = <global>.parseInt(this.targetModel.doSomething('getAbsoluteScale'));
					maxScale = <global>.parseInt(this.targetModel.doSomething('getMaxScale'));
					minScale = <global>.parseInt(this.targetModel.doSomething('getMinScale'));
					target.sandbox.initBtnShowHide(false);
					this.setVariable('VAR_ENABLE_ZOOM', true);
					this.setVariable('VAR_ENABLE_STYLE', true);
				}
				scale = scale - this.commonMinScale;
				minScale = minScale - this.commonMinScale;
				maxScale = maxScale - this.commonMinScale;
				this.setVariable('VAR_RADIO_FONTSIZE', scale);
				this.setVariable('VAR_MAX_FONTSCALE', maxScale);
				this.setVariable('VAR_MIN_FONTSCALE', minScale);
				this.setVariable('VAR_UPDATE_FONTSIZE', false);
				this.setVariable('VAR_UPDATE_FONTSIZE', true);
			}
		}
	};
	this.pageSizeOverlayModel.initModel = function () {
		var model;
		model = <global>.kbook.model;
		if (model) {
			switch (model.STATE) {
			case 'PAGE':
			case 'DICTIONARY':
				if (model.STATE == 'PAGE') {
					this.setVariable('VAR_ENABLE_ZOOM', true);
					this.setVariable('VAR_ENABLE_STYLE', true);
				}
				else {
					this.setVariable('VAR_ENABLE_ZOOM', false);
					this.setVariable('VAR_ENABLE_STYLE', false);
				}
			case 'PICTURE':
				this.setVariable('VAR_RADIO_FONTSIZE', -1);
				this.setVariable('VAR_MAX_FONTSCALE', -1);
				this.setVariable('VAR_MIN_FONTSCALE', -1);
				this.setVariable('VAR_ENABLE_ZOOM', true);
				this.setVariable('VAR_ENABLE_STYLE', false);
			case 'MENU':
			case 'MENU_HOME':
			default:
				this.setVariable('VAR_RADIO_FONTSIZE', -1);
				this.setVariable('VAR_MAX_FONTSCALE', -1);
				this.setVariable('VAR_MIN_FONTSCALE', -1);
				this.setVariable('VAR_ENABLE_ZOOM', false);
				this.setVariable('VAR_ENABLE_STYLE', false);
			}
		}
		return true;
	};
	this.pageSizeOverlayModel.zoomChange = function () {
		this.zoomModel.onZoom();
	};
	this.pageSizeOverlayModel.doNext = function () {
		if (this.targetModel.overlayModel.onNextInstantZoom()) {
			return;
		}
		this.targetModel.doSomething('doBlink');
	};
	this.pageSizeOverlayModel.doPrevious = function () {
		if (this.targetModel.overlayModel.onPreviousInstantZoom()) {
			return;
		}
		this.targetModel.doSomething('doBlink');
	};
	this.pageSizeOverlayModel.doRoot = function () {
		this.closeCurrentOverlay();
		this.targetModel.doSomething('doRoot');
	};
	this.pageSizeOverlayModel;

6.4.2012 after some thinking:

SizeOverlay is loaded and initialized each time you press the size-button.
So your xml-variables will get initialized too.
If you look in the function initSizeMenu, in the spoiler above, you will notice this.setVariable('VAR_RADIO_FONTSIZE', scale); where scale is a variable outside the xml, preserving the state.

So you will have to add simillar to your JS code.
Define a var to hold your custom value lets name it myFontSize,
write a function myInitSizeMenu with a line

this.setVariable('VAR_RADIO_CSS_FONTSIZE', myFontSize);

and hook that new function to initSizeMenu, so that it will be executed right after.

Core.hook.hookAfter(..context to be found.. pageSizeOverlayModel, "initSizeMenu", myInitSizeMenu  );

Another notice:
the sending object is passed to every button-action.
So there is no need for different functions doFontSize5/7/6/8

You can go for: doFineFontSize = function (sender)
and within check for sender.id. It's the id defined in the xml.