Hi there! I'm a second year undergraduate studying the MPharm degree at Cardiff, so hopefully I can give you some help!
I wouldn't say that Cardiff Uni is too big - most of the Uni buildings are very close together, apart from medicine and other healthcare subjects which are further away (about 40 minutes away from the SU) at the Heath Campus. Pharmacy is in the centre of things. Being a city Uni rather than a campus Uni has its benefits - there's a lot to do in town, and the shops are close to the Uni. There are two other big uni's with buildings in Cardiff, but personally I find that this means there are loads of cool new friends to meet with, which is a great thing!
There are plenty of societies to get involved with: look them up on cardiffstudents.com! And if you don't like any of them, you can always create your own!
ALL types of people go to Uni, older students, overseas, those concerned with purely studying, those who like to party - it's a massive mix. You will be sure to find some like-minded people on the course.
The lecturers are very friendly and helpful at Cardiff. There are a few who could be more organised, but you'll find that wherever you go. I stayed in Senghennydd Hall, which is part catered and I really liked it there, But Talybont Court is also nice and if you want to be super social, Taly North and South are the places to go. There are actually relatively few Welsh students at the Uni, which I think is a shame! But there is a massive amount of diversity, which is great! I would say the worst thing is the stress of finding a student house after the first year - letting agencies try to take advantage of you, some of the houses are disgusting and people very often have problems with housemates. But again, most unis have this problem and at least Cardiff is cheaper (due to the massive student population) than other places!
Hope that helps! x