how do you change the font size in a text box?

You change by:
<font size=#>
Replace the# with whatever size you want the font to be.

I hope this helps.

OR
<font style="font-size:#pt">
if you want it a little more exact

If you mean a text area, they don't allow HTML. Your code would look something like this:

<textarea rows="5" cols="20" style="font-family:verdana; size:9pt;">Text here</textarea>