I'm at Northumbria University living in Camden Court halls. The ROOM size is pretty decent, enough to fit all of my stuff from home in though it's not massive and when it's untidy it does feel small. The desk is massive though and lots of space under the desk too, as well as big drawers and space at the end of the bed. I have an en-suite which is a reasonable size, obviously small but there is enough space to sit on the toilet, stand at the sink and use the shower without feeling squashed or cramped. I have 5 flatmates who all have similar rooms and we share a kitchen which is actually a pretty decent size. We have a big fridge/freezer and then an additional fridge between us, plenty of cupboard space, two sinks, a table with 6 chairs and another 6 'lounge chairs' near the big windows which are basically lower down chairs with a cushioned back.
Accommodation does vary though, I know some people have better rooms and kitchens than me but others have worse. I'd strongly recommend looking around THE ACCOMMODATION you intend to apply for so that you can see what it's like.
Also even though you're in a flat with other people you will always have your own bedroom if you want to be on your own sometimes. Very few universities have TWIN ROOMS where you have a roommate as well as flatmates but it is very rare and you don't have to accept accommodation you're not happy with. I've only heard of one university doing it anyway and I think that was Aberystwyth, though they have single rooms too.