Put simply, the longer the crane, the more it'll bend the mast because of its leverage.  An estimate of its leverage is given by imagining where the backstay would attach to the mast if the crane were not present, yielding an "effective" mast height.

Usually, you'll not want a particularly long crane;  in fact you'll probably want the shortest crane that will allow the backstay to clear the mainsail leech.  The reason is that you'll probably want as much backstay tension as possible, so as to obtain as much jibstay tension as possible, while bending the mast as little as possible.  If you are running a minimum diameter mast, you'll probably want to be running a minimum length backstay crane as well.

A small detail:  to maximise the transfer of the tension in the backstay to the mast, have the crane meet the backstay at a right angle.