You can do cab missions by going to your cousins cab company and takin one of this cabs, or something like that. Just read it in a review for the game.
I think it's one of those games where it can be as short or as long as you want it to be, depending on how much shit you want to do/accomplish in the game.
how is the length of this game compared to Vice City and San Andreas? I'm not talking about doing every side mission...but probably the average amount of time to beat the game
I don't think it'll be superlong. A LOT of people never finished main storyline for San Andreas cause it got repetitive for them (I concur, I prefer GTAIII over it and vice city).