My hands are about average size, maybe just a little smaller, and I do in fact have difficulties reaching the thumb sticks.

It's not a major issue. I've had my Deck ever since original LCD launch, and absolutely love it. But playing on something like the Legion Go S is noticeably more comfortable.

Overall, it's probably not about hand size, but grip and preference. There are many people with small and weak hands who say the Steam Deck is perfect for them. And there are also people with big, strong hands who say that it's a little too big or heavy.