Hi Mike,
the absolutely, positively, correct way to do this is;
a) add a class to your custom.css file
.whatever{
font-size:16px;
}
b) in the template set the field to use css class
' whatever'
(not the leading space, which you probably want)
I know this seems like the long way around, but just do it right - you'll thank me later.
cheers
Bruce