I like the "half" sized ones. They hold enough for a weekend+, and still allow me to keep the stuff that stays in my hard bags in there. (Jumper cables, some tools, hat, spare riding gloves, etc.) Even the regular ones can be filled enough to take up most of the available space inside the hard bag.
And I still manage to pack more stuff than I need... Laptop+charger, tablet, extra USB chargers for cameras/phone/etc., small extension cord (motels never have enough outlets available for all my electronic crap), etc...