When I first got here I thought it was very small, but that's because I was comparing it to all the cities I've lived in before. Compared to other college towns I think U-C is pretty nice.
http://www.dailyillini.com/article/2014/10/champaign-urbana-ranked-7th-in-top-college-towns
As for the neighborhoods, I like Urbana better than Champaign. I live close to downtown Urbana and it's very nice and pretty quiet, and also close to campus. But that's just my personal opinion.
I think you'll meet very nice people here, but I think you can meet nice people everywhere! Well, undergrads are definitely younger than 29, and I'd say most grad students are too, but I've met quite a lot of people in their late 20s or even older as well, so that shouldn't be a problem. I wouldn't say we're a majority, but we're not a rare species either!
Chicago is pretty close (2 hours and 30 min. drive) so it is definitely possible to go there in the weekends.
Saludos!