Bill,
The size of the mold depends on the way the model is being made. Injection cast molds are larger than resin cast molds. I make the masters for a line of resin cast models, In many cases the molds are only slightly larger than the models. In resin casting the resin is poured into the mold, air bubbles are drawn out and the resin sets. In injection molding the casting material is actually injected into the mold. There are advantages and dusadvantages to each.One of the biggest advantages to resin casting is that it is less expensive to make a master than in injection casting. The problem is that resin casting is a slower porcess and the molds have a much shorter life. It is often necessary to remake a mold in this process. Injection casting is a much faster manufacturing process and becasue the molds are metal rather that a rubber type material they last much longer.
I hope this isn't more then you ever wanted to know but you did ask. :0)
Gerry Siegel
Mountville Pa.