GTA EFLC was about 16Gb, i believe GTAIV was close to that size too. Clearly GTAV should be bigger considering bigger map and textures, and also considering today's standards. For comparison to a more recent title, Wolfenstein the New Order is a little over 45Gb if i remember correctly.
Edit: Also keep in mind Max Payne 3 on PC was about 35Gb.
With all that in mind, who dares to guess how big the game will be once installed on your HDD/SSD?
Feel free to vote and type an specific size. I'm going to say 44Gb.