All 3 are fairly large, though spread out very differently.
Vancouver: Basically a square, made up of 4 smaller squares of different elevations with moderate cover. Note that while you can use the air ducts as cover enemies can shoot "under" the cover and hit you, so its risky. Generally people like to start at the high-ground near the labs and work their way outward, the labs has the best cover in general and good sightlines.
Rio: Long skinny map generally divided into 2 halves by the control tower in the center. Due to it's nature it is often easiest to start on one end and work your way across, but many players want to hang out in the control tower. This map also has a whopping 8 ammo crates, 2 of which give 3 grenades.
London: A + shaped map with a clear high-ground and killbox, fairly simple to control, but be warned the killbox works both ways, and as such it is dangerous to enter if you don't have a clear idea of what will be shooting you.
Edit: As to what classes work best on them... the only thing I would say is melee focused characters have a moderate disadvantage on London due to the killbox nature. Other than that no classes are particularly better or worse on a given map, they all have areas suited to any playstyle.