Thank you for your post.
At this time, this is controlled by iOS, and FileMaker Go does not have access to those settings and lock a particular setting (in your case, "small").
I encourage you to enter this suggestion into our Feature Requests web form at:
The entries into this web form populate a database file that is hosted and monitored by our Development and Product Management departments where the entries are then discussed and considered for future releases. Although I could easily copy your post and paste it into the web form, there are a couple of questions on the web form that only you can answer.