For now i can say, that the cases are solvable in 30-90 minutes, depending on how long you take to investigate the crime scences. but i wouldn't rush it, 'cause its pretty enjoyable to actually search for clues and such.

there are some side missions were you hunt down criminals either shooting them down, or giving them a good beating. also there's a lot to collect from hidden cars, famous places, newspapers to filmreels spread across town.

size of the town is huge i'd say at least as big as liberty city, i guess i've only seen like 5% of the town yet