As of posBoss iPad app version 4.7.2, there are three sizes you can select from in the printer settings (excluding Epson Dot Matrix which has only the medium size available)
To change the font size, head to Settings > Printers on the posBoss iPad app and select your printer.
Here you can select Small, Medium or Large. The pic below will give you an idea of the size difference for a Star or Epson Thermal. Give them a go on your own printer and decide what is right for you.