Being a stock-removal guy, I'm talking completely off the top of my noggin here... but I'm sure there's a geometric rule of thumb for determining the final mass of the piece you want to end up with, and thus select the appropriate mass of steel to start with. It seems to me the biggest advantage of forging is making the very most use of the steel you start with. (I'm sure you've noticed that most SR guys simply don't make a whole lot of big Bowies or camp knives... it gets pricy out of proportion to the finished product really quick. I "waste" a stupid amount of perfectly good steel, time and belts when I grind a big honkin' chopper out of 1/4" stock... a forger who knows what he's doing and how much steel he actually needs, wastes very little, whether he starts with bar, rounds or spheres.)
On the other hand, the OP may be asking how close to final dimension you all forge, and how much scale you expect to grind off after HT to get back down to fresh clean steel.