Let's forget about guns for a second and think about duty footwear. Supppose you went to the Chief with this proposal: "Chief, I've averaged the foot size of every officer on the department and we can save a ton of money by just ordering footwear in that size."
You'd be laughed out of the Chief's office with good reason. There is no way that one size can fit every officer on your department. For some it will be too small, others will find it too big and in some cases, the size problem will be so significant that it affects job performance.
This is the exact same problem that you find when you try to standardize on ONE size of duty weapon. There is no way that one pistol can fit every officer, especially with the rise of female officers.
I'd highly recommend an "approved list" of duty weapons from major manufacturers and allowing your officers to choose a weapon from that list. This ensures that officers are invested in their weapon selection and will likely choose a weapon that fits their hand.
If this seems logistically impossible, I'd look at the Las Vegas Metropolitan Police Dept. The agency has roughly 2000 officers and allows all of them to choose their duty weapon and caliber (9, 40, or 45) with great success. They do have weapon they will issue officers that won't buy a duty weapon but it is a second generation S&W.
At the least, you can standardize on a weapon system and allow your officers to pick one model. For instance, my agency requires that you carry a Sig classic. The particular model, 220, 225, 226, 228, 229, 230, 232, 239, or 245 is up to you and you pick the caliber (9, 40, or 45). This allows our armoers to only know one weapons systems but allows us to accomadate concerns such as hand size.
Forcing officer to use only one weapon is a waste of resources as it is impossible for one weapon to fit all officers. Give 'em some choice and they'll perform better.