Size 1 is too small.

That's smallfont size. Make it size=2 and it should be fine.
You don't need a hack to let users choose their font size. Make a custom profile field, get your users to enter the font size they want in it in their user CP Profile, and then in your postbit template, have this <font face=Verdana size=$post[fieldx]> and </font> with your $post[message] in between (x being the custom profile field id). There you go!