On the form field properties, go to the "Appearance" tab and set your font size and font. If somebody only has Reader when they fill in the form, they cannot modify this information. For somebody with Adobe Acrobat, you have to do a bit more: You can apply document security settings that prevent them from modifying the document. You do that in Document Properties (Ctrl-D or Cmd-D), and then go to the Security tab. Here you apply "Password security" and then use the "Restrict editing..." option. Make sure you pick an option that allows "Form filling" - you of course also have to allow printing:
Form fields are locked to a specific font and size, unless the users are editing them using Acrobat or if you've set them as having Rich Text Formatting.
If you did the latter, then don't. If it's the former and the users are editing those fields then you can apply a security policy to the file that will allow them to fill in form fields, but not edit them.
Well, that was embarrasingly easy. Guess I needed to look at it with glasses on! Thanks Karl Heinz (and try67)!