in Brickell it's mostly the big skyrise condos, with some smaller apartment buildings (though everything in brickell is fairly tall). I wouldn't say they're particularly small, at least compared to the other major cities I've lived in. There's some variance, but for example I live in a 2/2 that's about 1000sqft, which is more than enough space. My apartment seems like a mansion compared to my friends NYC shoebox... Definitely pricy, which is why having a roommate is basically a requirement if you don't have support outside of loans. 2 and 3 bedrooms are wayyyy more affordable than a 1br or studio would be.
Brickell isn't for everyone, but there's a reason a lot of people (especially first years) live there. To some extent, you're getting what you pay for - there are more affordable places in Miami than Brickell, but you're making tradeoffs. Brickell is close to other students and restaurants/bars/etc which helps a lot with your social life. It's generally pretty safe. It's on the metro, which makes it easy to get to school and a few other places (the Miami metro is very limited, so many neighborhoods aren't served by it). And it definitely doesn't hurt to wake up in the morning to a view from your high rise window of the sun rising over Biscayne Bay...