LMB generally feed on BG about 1/4 to 1/3 their length... For examply, a 12" LMB would be eating BG in the 3"-4" range. Not to say they will not feed on smaller BG or perhaps one a hair bit bigger. Generally though, this is the ideal size.
As Bill said, LMB growth is highly variably. In new ponds, one can expect LMB growth to be exceptional. LMB pushing 10" is not impossible. It depends on the size of the fish stocked as Bill said. Let's assume a typical stocking length of 3"-4". I would best most of your LMB will be in the 6"-8" range with a few pushing 10" if your forage base is suitable.
