That "A" is hard to tap with the right angle and pressure - I use an Apple Pencil on iPad Pro.
You can also go in to either Full Score Settings or Parts Settings (Depending on which you want to change), select Edit Titles and Text, then the specific item you want to change. At the bottom of the information is "Font" - follow that & you can change the font & its size.
There are some elements I haven't found a way to change in iOS, but can be changed if you transfer the score to Mac. Measure numbers, for example. I find that they are much too large, especially if you increase the size of the notation font for aging eyes. I like to print in Notion's notation size 6 to make it easy to read, but the measure numbers which I like on every measure are far too large unless I move things to the Mac and reduce their size before printing.
