Ah, I understand now. Thanks for making that clear. I thought you had found a new way of adjusting the font size in an .epub file.
Whenever I stipulate a font size in my html documents and then convert to .epub, I find I do not have any option to change the font size at all on my Sony PRS-505.
I usually omit the font size (just stating the font-family), convert with Calibre to .epub and then the Sony gives me the choice of 3 font sizes.