With respect to your friend, they don’t know what they’re talking about. Or, at least, I don’t know what they’re talking about. Full frame Glocks have grips that are roughly the same size as most modern handguns.
Thing about hand size is, most of these weapons are designed to be used by any adult. You’ll get some outliers with really tiny hands that can have a problem. But, these weapons are designed to be usable. They may be slightly less comfortable for your character, but six inches shouldn’t be enough to make anything downright unusable (outside of maybe the Desert Eagle… which, isn’t really a solution to any problem aside from “how can I get a 4lb gun plated in chrome?”)
I still have a hard time recommending Glocks, but that has more to do with how tightly you have to pack the magazines. Fact is, in spite of them being specced for 17 rounds, I’ve almost never loaded a Glock 17’s mag past 15, because life’s too short for my thumbs to heal from that crap.
That said, Glocks do have you covered. You can get Glocks in .40, .45 and 10mm. 10mm is a weird and absurdly overpowered cartridge that never quite caught on. For the most part you can also pick the format. Glocks exist in Full Frame, Compact, and Subcompact. Subcompact Glocks are tiny. They’ll also frequently accept magazines from different Glock pistols firing the same cartridge. So you can load a full frame magazine into a compatible sub compact pistol. You can search online to find the specific number that goes with your preferred cartridge and form, as well as which magazines that pistol will accept.
Personally, I really like H&K USPs, because of the blowback counterspring system, which eats recoil like you wouldn’t believe. Also, like Glocks, you can get them chambered in .45. I don’t believe you can get a 10mm USP, but you could probably get a gunsmith to convert a .40. I’m just guessing there. There’s a compact variant, though that’s trading ammunition capacity for form factor.
The Walther P99 might be a good fit. Though, there isn’t a standard .45 variant. (Though, I think S&W does sell a licensed .45 P99.) They’re actually nice for people with smaller hands because they ship with three backstraps, so you can adjust the pistol to the size of your hand. I’ve never found them comfortable, but that probably has more to do with the fact that all of the ones I’ve handled have been rentals, and I couldn’t swap out the backstrap.
SIG Sauer P220 series pistols are another classic. The P220 is available in 9mm (avoid this) and .45. It’s got a single stack mag, and it’s a nail driver. The P226 is externally similar, but has a double stag mag (15 9mm rounds). 226s tend to jam on me, but that’s not a normal issue, and these are very reliable. The 227, 228, and 229, are compact and .40 variants. Though, I don’t keep the specific numbers straight. These are standard issue for a lot of law enforcement agencies, including the FBI, and a few national militaries.
Not nearly as flashy, but there’s the S&W 4506. This is a heavy steel frame .45. The entire 4500 line of pistols had homes in law enforcement, but I’m singling this one out because of the cartridge. These got phased out in ‘99, though, so the ones you’re seeing today are at least 15 years old.
These days S&W also has a series of M&P pistols. Or Military and Police. I’d recommend looking those up to see if that fits what you’re after.
Modern 1911 pattern pistols come in nearly all flavors, but, if for some reason your character really needs a 10mm or a .50, somebody probably makes one. Just be aware that a .50 M1911 is probably going to be a bit bulkier than a .45.
(Michi Note: I’m 5”5 and have really tiny hands, tiny like I have difficulty stretching to hit the six button on the keyboard tiny. My experience with the Glock was: it’s uncomfortable, not impossible. I can use it if I have to, but it’s not going to be my first pick at the gun range. For comfort, I prefer the P99 and the USP. However, your character probably has larger hands than mine as most men do. So, he’s good.)