In general, yes, but it's not easy to do with the precision needed to guarantee a perfect fit as you can count the number of characters in the field, but that doesn't tell you how much space they take up and this will vary with each letter unless you use a monospaced font instead of a typical proportional font.
You can set up a conditonal format with an expression like this:
Length ( Self ) > 30
and then specify a smaller font size. You'll then see the text get smaller when you enter more than 30 characters.