Change Field Name Font Size

The only settings are per user.  You can go to your name at the top right and My preferences and choose "Relaxed".  That might help. 

It's sort of like buying blue jeans, you always hope to fit into the skinny jeans but some of us need "Relaxed" depending on your screen size and eyesight.

Mark would be correct that the size of fields in the form don't have a preference that can be set by the app builder, it is reliant on the user to select "Relaxed". Alternatively, knowing how much time I spend in QB and on a screen I have my chrome set by default to be zoomed in on QB windows at about 125% to help with readability if that helps at all with making reading forms and reports ab it easier.