Personally I wouldn't go for the largest bag available unless you absolutely need the space.
The largest bags are meant for people who want to stuff multiple boards and literally all their gear including base layers and clothing inside the one bag.
The space is nice but you have to remember larger bags means not only does the bag itself weigh more (2 to 3 kg more), but you also have to be careful you don't pack it beyond your free allowance if you travel via air.
I'm a fan of something medium sized, not too big but not too small so you have the room available if you need it. Medium sized bag will fit 1 set of gear including your bindings (2 sets of gear if you stack the boards up) and have some room for mid layers and other outerwear as well. It's a nice middle ground and this way you get that extra 2 kg of precious weight allowance when you travel via air.