I would post this in the lettering thread but I'll post here so you don't miss it...
As I've been told several times it all comes down to the font you choose but from my experience, Unmasked from Blambot works well at 10pt on an A4 page. If you type out a balloon using this font and size then compare it to the font you are using, you should be able to work out what size to choose.
Hope this helps.