I don't know how effectively waterproof the bag is, but it only takes 2 twists before you notice air getting trapped inside. You need to attempt to squeeze out all the air before folding the bag to prevent it blowing up like a balloon. I use a 15 litre bag for my Medium sized 2.5mm summer weight shorty, and there's plenty of room left. I could easily use the 10 litre version. If you have a heavy weight wetsuit I'd suggest the 15 litre option, the bag certainly keeps the rest of my kit dry when it contains a damp wetsuit.

38x20cm.