Hi, I'm a Balliol fourth year.
I applied for mid, and was put in low anyway, so it's pot luck really. The rooms vary a lot - my first year one was small (about 15m^2) but decent, with a sink, desk, wardrobe, shelves, and enough floor space to walk around a bit, but some of my friends got massive rooms. there tends to be 2 toilets and a shower or two between 8-10 people, depending on the staircase, but that will be the same for any category (unless you get an en-suite one, which apparently some are).
My advice would be apply for mid, and you'll probably get low, as there are a lot more of them, but it'll probably be near the upper end. However, if you have limited money just go for low. If you have unlimited money go for high, because the high ones are massive (25m^2 or so).
It doesn't matter too much what you have in first year - it goes so quickly, and rooms in second and third year are much better anyway, and you get to choose them personally.
We have formal hall (which is once a week, and £13, so expensive), BUT don't wear gowns. I love my college, but that's one thing I don't like, so if it's a deciding factor for you, another college may be better - Merton and Christ Church are famous for it, but most colleges do it.