What do you mean? In Acrobat Form (acroform) fields? In Acrobat Pro DC (in Windows, but assume Mac is the same), go into Prepare Form mode, select the field (or

What do you mean? In Acrobat Form (acroform) fields? In Acrobat Pro DC (in Windows, but assume Mac is the same), go into Prepare Form mode, select the field (or control select for multiple text fields), right click Properties, go to Appearance tab (Text section), and the either select Font Size: Auto (for autofit) or the point size you want.