Although I don't use the iPad Forum app myself, I thought that someone else might have responded to your question if there was a way to do this. I looked back through the old posts on the app and several people have asked for this in the past so it appears as though it's not possible.
Of course, you can always simply use the accessibility features of the iPad to permanently increase the font size
'Settings', 'General', 'Accessibility', 'Large text' ->'On'
but this will do it for all apps and not just the iPad Forum one.
Tim