In my experiences of living in (undergraduate) accommodation I've found that the sort of space issue you're concerned about ends up not being too much of a problem for various reasons. In my first year I lived in a small room on main-site, but I was close to both Hall and the JCR, which provided plenty of communal space to eat and meet people. Often I'd eat breakfast in my room (my college lets students have a kettle and a toaster and mini-fridge if they want - I didn't have a fridge, just kept milk on my windowsill
), which meant that I eat quickly and got on with the day! I found there was plenty of space to study, sleep and entertain people if I wanted to.
In my second year I lived in off-site accommodation which had eight student rooms in one flat with a small kitchen and a table that we could all (just about) fit round. The way the flat was organised it didn't really lend itself to bumping into people unless it was in the kitchen, but we saw plenty of each other at mealtimes and generally had a good amount of human contact!
I think something to remember is that Oxford is a pretty compact city** and you'll probably end up spending a lot of your time in libraries. There are so many different libraries in Oxford that you'll never be bored when it comes to study spaces!* If you're savvy about it you can also figure out which ones have good canteens or just coffee machines - I think the coffee machine in the Philosophy faculty charges 20p for a mocha.
You don't have to be a faculty member to use a faculty library so you can go anywhere with your Bod card!
If your accommodation is close enough to your college you'll probably also end up going to Hall a lot - it's a good place to socialise and also generally normal (i.e. non-formal) college food can be pretty cheap (and in some colleges even formal is ridiculously cheap). I'm always aware when I go to Balliol Hall that I could definitely cook myself something tastier, but could I do it for less per meal? Probably not! Your college might also have a bar and will definitely have facilities for MCR members (this can vary from a single room to a whole complex, in my experience - the Balliol MCR has a bar, tv room, big kitchen and large space for watching films on a screen / hosting parties, plus an oak-panelled MCR***
Basically, I really wouldn't worry about feeling hemmed into one small room for your whole time at Oxford! St Aldate's, if that's where you end up, is so close to town -- I think you'll end up spending a lot less time than you'd guess in your room!
*You can choose between the carved marble of the Radcliffe Camera, the austere portraits staring down at you in the Old Bodleian, the ridiculous vastness of the Codrington, the cosiness of the Haverfield Room in the Sackler...
**I stopped moaning about the five minute walk to Exam Schools for lectures when I realised that some of friends have to catch a bus to get to their lectures at other uni's.
***Oh goodness, the irritating confusion that arises from the room and the organisation having the same name.