The size of the USB drive should be enough for the entire disc. Setup each DVD like this.DVD1:This DVD has to be installed on your console's internal HDD or a USB drive which was formatted using your console.
You need this dvd to play the gameDVD2 = Single player + GTA OnlineWhich means the first dvd is just a normal DLC/additional installation and not an actual game.2. The contents of each dvd are this:DVD1 = Mandatory content Installation.