I am 100% sure gta V will be at least 2 times bigger than the gta IV map although it(gta V) has only a single city (los santos) and surroundings(mountains,lakes............)
gta IV was about 14 gigs and seeing the size and graphics quality of gta V, gta V will be about 25-30 gigs and will be released on a blu-ray disc i suppose.
whatever......i am sure rockstar wont dissapoint us they need time in order to finish such a beautiful piece of computer tech,animation and common sense(combined)
it is in final stage of development and will come out soon after some screens and one or two trailer(s).
rockstar isn't announcing release date because they fear DELAYS like in case of max payne 3 and gta IV rockstar has learnt from its past experiences
and the people in rockstar are working really hard ,so have patience and after release have a lot of fun... :D :D :D :D .