Toby is absolutely right.  I am fortunate enough to have been on a few driven days, though I have done more 'walked up' grouse.  I have seen all almost all guns from 25" Churchill style used to good effect through 28" side by sides to 30" over and unders.  I would say 28" side by side has been the most common size I have seen.
I knew one person who every year took a double 12 bore Joe Manton muzzle loader on a walked up day over pointers.  It filled the bag and the owner had a wonderful time using it.