Looks like set CSS style > "font-size" doesn't affect the TextBox object at all. "color" and "border" does.
Example:
dl.dropbox.com/u/13211981/textbox_css.capx
r100 beta
Windows 7 64-bit Chrome/Firefox/IE
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Not a bug: you have 'auto font size' set to 'Yes', which will override the font size.
Of course... Sorry for posting "bugs" all the time;)