No offense, but, that does sound pretty bogus.
Harley is 5 months old. Several vets have said that they dont perform the surgury until 6mos. of age since they have to be put under anestisia (sp?) (No ACE!

) But, I know some people who have had their boys neutered at 4mos. of age and they are VERY masculine. I dont think it affects their masculinity or size. So what if their a few lbs smaller than they would be if left unneutered. For the best intrest of your pup and his health, I'd get him neutered way before 1 yr. of age.
Also, as far as "keeping him away from little girls" When Boys are teenagers (and yes, our furboys are teens at a point too) they will jump through hoops to get to a female in heat, no matter how hard you watch them... they will find a way, after all they are males!
I'd call around to Vets in your area and get several opinions before waiting too long.
Good Luck
Melissa