[ecostin]

Amazing! Amazon plays fair, see below:

public float[] Sy() {
    J b;
    float[] a = b.YTD("/mnt/us/FONT_RAMP");
    if (a != null && a.length > 0) {
        return a;
    }
    return (float[])ResourceBundle.getBundle("com.amazon.ebook.booklet.reader.resources.ReaderResources").getObject("font.size.mapping");
}

public float[] zy() {
    J b;
    float[] a = b.YTD("/mnt/us/CJK_FONT_RAMP");
    if (a != null && a.length > 0) {
        return a;
    }
    return (float[])ResourceBundle.getBundle("com.amazon.ebook.booklet.reader.resources.ReaderResources").getObject("font.size.mapping.cjk");
}

This means that font sizes can be set without jb and without any patches, by creating said files (FONT_RAMP, CJK_FONT_RAMP) in the root of the USB partition. The content must be, for each file, exactly 8 float values, which correspond to the 8 different font sizes:
[root@kindle log]# cat /mnt/us/FONT_RAMP
7.49 8.43 9.37 10.31 11.25 12.19 13.13 14.07

It works perfectly, although the size of the characters in the font menu (where one taps to select the size) do not change, they stay the same.

A framework or device restart is required after creating the files.

I did not check yet if this works for non-kf8 books.