I think the short answer to this is the largest size that you can comfortably carry. If you're under about 5’5”, the larger bags are going to have shoulder straps and waist loops that may not fit your body very well.
How long are the walls you're planning to climb? Are you heading up a short outing in Zion National Park, where you're probably going to be on the wall for maximum one night? Then you can use a smaller bag. Or, are you heading up El Capitan on an outing of more than about three days? Then you probably want the biggest bag you can manage.
Haul bag sizing and pricing is kind of an interesting situation.
Let's look at Metolius, one of the most popular brands. Their largest two bags are called the El Cap and the Half Dome. (Costs are as of Feb 2019)
El Cap: 157 liters, weight 3.6 kg, cost $229
Half Dome: 125 liters, weight 3.4 kg, cost $219
So, for only $10 more money, and a weight increase of only 0.2 kg, you get an extra 32 liters of storage! For me, that's a no brainer. I'm getting the El Cap.
Below: the Metolius haul bag line up. From left to right, Half Dome is #3, El Cap is #4.
Having some extra room in your haul bag, rather than packing everything into it like a Chinese puzzle, can make your life a lot easier.
And, of course, you don't want to fit everything in your haul bag anyway. For things like your portaledge, garbage bag, poop tube, wall bucket and your day bag, you probably will be using gear tethers anyway, like we described on this tip.