I have a large litre water bottle in my main run, where I usually have 5 piggies. They are all mature and able to deal with the larger spout. In a day they probably get through about a 1/3 of it - although it does tend to drip a bit so it's hard to know how much is in the piggies and how much evaporating!
At the moment, I have 2 piggies in the 'hospital wing' (Truffles has an abscess and Muffin's in there for company!). They have a smaller 300 ml bottle because I'm feeding them pro-biotics. The smaller bottle doesn't seem to drip as much as the big one, and they get through about 150 ml a day between them.
I'd get a bigger bottle if you find that you're having to fill up more than once a day, otherwise they're probably fine with what you've got. They important thing is that they always have access to fresh water and that you keep they're bottles clean.