I owned a Mark 23. Although it is physically a large pistol, the grip size, trigger pull is almost the same as the USP grip. I have average sized hands and I had no issues shooting it, or the DE .44 Mag I owned at the same time.
I am like several other members above. I purchased a HK45 about 2 years ago to add to my Mark 23 and USP .45. I found I preferred the 'block like' grip 'feel' of the Mark and USP over the sculpted HK45 grip.
I ended up selling the HK45 after a few months as I liked the feel of the USP .45 and I could shoot it as well as, or sometimes better than the HK45. As mentioned above, the HK's share the same internals and are well known for their accurate, soft shooting ability.
I sold the Mark 23 as it became a 'safe queen'.
I own a HK USP Custom Sport, which is on the EE now, mainly because it has also become a 'safe queen', as I have several other 'go to' 9 mm's to shoot, a German made Sig P226 and G17 with custom trigger. The USP Custom Sport is basically a USP Tactical without the threaded barrel end.
Unless you really 'want' a .45 pistol, I would consider a HK USP 9/.40 pistol, as the grip is slightly smaller than the USP .45/Mark 23.
Also, I'm not sure what type of shooting you do, but between a 9 mm and a .40, for the type of shooting I do, I would stay with a 9 mm, as the recoil from a 9mm HK is very soft, plus the 9mm is cheaper to feed, making it easier to practice more.