35ltrs per side is about right for panniers, you can go bigger but dont forget a few things;
if you have space you will fill it with something you will never need, thats a scientific law
you may have to pick the bike up at some stage, so you dont want them too heavy
you dont want them too low or they will bottom out in deep ruts, possibly ripping off, buckling, or if you make them strong enough, pull the whole rack off the bike!
dont put them too close to the footpegs, and agree with oldbmw about sharp edges near your feet. if you mount them up level with the luggage rack/pillion seat its always handy to have a broad flat area to strap stuff to.
otherwise its your bike mate, use your imagination and have a great trip!
35ltrs is a good size, i personally like a topbox too