Hi A, OK In the position your talking about the your penis would seem smaller, being bent over, unless your measuring it another way, as for lasting 10-15 minutes, there's probably a whole bunch of guys out there who would love to last just the 10 minutes.
But I would try just doing something different from time to time, as they say a change is as good as a rest, as for harming your sex life, don't think so, but just doing it all the time you both find boring.
Good Luck
Do you mean she masterbates you?