The vagina is very adaptable.
Consider what happens during childbirth.
Thus, in reality a guy can never be too big.
Now of course in the normal course of sex there is the matter of lubrication as well relaxation and expansion.
The amount of tightness for either party is really what it is all about, it translates to feel and especially if the right points are involved in the "mechanical meshing" (for want of the correct term which eludes me at the moment) all should be happy and hopefully both partys will experience the desired orgasmic ending each is seeking.
Although in a monetary transactional setting different results are probably being sought.