Question: Change the font size in a fillable PDF form
I asked this question and found the article change font size in fillable PDF? running Safari 11.0.3 on Mac OS 10.13.3.
My solution has been to use Readdle's PDF Expert (version 2.3.1 at the time of writing).
The software automatically reduces the font size as you type to suit the space in box on the form.
It's not freeware but it's less expensive than Adobe et al.
I've found PDF Expert extremely useable and useful.
MacBook Pro (13-inch Mid 2012), macOS High Sierra (10.13.3)