Hand fit versus comfort
I would consider what your needs are with regards to "fitting the hand" and try this the next time you are able to go to that pistol buffet. By the way $1 rentals- I wish I had that near me, sounds like a lot of fun since I love to experiment.
BARE NECESSITIES of fitting the hand vs comfort
Here is something to try if you suspect a pistol is too large.
If gripped in 1 or 2 hands are you able to actuate the side release and safeties with your thumb? If so that is good. If you relax your strong hand a little does the pistol threaten to tumble out or can the trigger guard rest on the top side of your middle finger? In other words for a larger pistol are you clinging on with muscular strength of your fingers alone or is bone structure helping? Can you reach the trigger with the pad of your index finger? Please note that for many the magazine release cannot be reached without a shift in grip and that is not necessarily a make or break for me.
For pistols that are possibly too small-
For all of us when we start looking at smaller pistols our gut instincts about what "feels good in hand" translates into "fills my palms completely". We are used to circular broom, axe, hammer handles, steering wheels with a round cross-section. This is not really needed to shoot accurately although it would feel better in hand. Having a gap in the palm of your strong hand feels strange but people get used to things rather quickly. You have stated you want a smaller pistol and typically this means an intention to carry so concealment takes a higher priority than a palm-filling-grip for many. That means on the thin side is good.
How do I know it is TOO small? If it is so small you cannot press the trigger. Try this: with a two handed grip (thumbs forward) can you press the trigger with the pad of your finger (or crease for a double action) without your weak hand blocking the motion of the trigger finger? Or does the meat of the thumb of your weak hand stop your index finger from moving? Even if you back out your trigger finger a bit?
Those are what I usually look at, while remembering that any new gun will feel strange to you until you have shot it, practiced safely with dry fires, draws, etc.
Specific complaints that you may not realize at a range session are things like: does my pinky get pinched when I change a mag, are the corners too sharp to conceal carry in my pocket, etc but even those can usually be addressed.
Sounds like you can ask the man behind the counter for "compact" sized pistols. And remember if you just love everything else about the gun, shoot well with it, size can be changed within limits with grip sleeves, some have interchangeable back straps, or you could even have grip reduction work by a smith or the adventurous.
My carry pisols vary from j-frame revolvers (can purchase any size grip) M&P40c, kahr cm9.
