Capacity obviously dependent on how much you want to cook at a time; we've got a 4.5l one (I think), which provides 6-10 portions of stew, so for us generally leftovers + some for the freezer. It's a basic Lakeland one, which I picked up cheap on sale, but it does have an 'auto' setting, which is good if you want to have it cook all day; it runs on high for two hours, then switches to low. More expensive models I've seen will also cut out entirely after a certain period, but I can't see the need in actual use. With my schedules I've never really done the 'get it started before work and come home to cooked dinner' thing, but it works well for days when I'm working from home, and/or have a late morning/early lunch prep window.
(ETA - given how simple they are, there doesn't seem to be much value in getting a more expensive one; I like having an auto setting, but that seems to be available on ones from £25 or so up.)