Let me ask you a question. Have you ever burned kindling? If so, you know how fast it burns and then goes out. So too small is a bad thing. Have you ever burned (or tried to burn) a huge unsplit piece of wood? If so, you know that it can burn forever, but you won't get much heat out of it. It'll likely just smolder inside your stove. So too large is a bad thing too.
There are a few factors that go into how big you should keep your split firewood and some of it has to do with your stove size. If you've got a tiny stove, you'll need to split your wood to match. I'd say about two to three inches thick would be perfect for a small stove. If you've got a large stove, upwards of ten inches is fine. Personally, I tend to keep my firewood under that diameter. Also, if my rounds aren't larger than six inches, I won't split them at all. So in large stove, the larger, the better. Just be sure that your wood is fully seasoned. These new EPA stoves don't like wet (green) wood at all, so don't even try to burn that stuff. If you've got two years to dry your wood outside, that would be perfect. I've tried to season my wood inside an open wood shed and three years later the stuff still hissed inside my stove. So frustrating.
The balance you need to strike is between dry time and burn time. We all know that if a piece of firewood is too large inside a wood stove, it won't burn well. So split that in half. We also know that larger pieces of wood (over six inches) that aren't split take forever to dry, so split those in half. The thing is, if you split your wood down to two or three inches thick in a big stove, you're going to be burning through it like crazy and no one wants to do that. It'll also burn hotter than normally which may make your house too warm. I think a good mix would be to split a variety and then bring those pieces inside. Start your fires with the smaller stuff and then add larger pieces as the day and night wear on.
But for all that's holy, please be sure to dry your wood long enough. Grab yourself one of those moisture meters and make sure the interior is under 20% moisture. And don't just test the ends or the outside. Split a piece in half and test the interior. I think you'll find that firewood takes a lot longer to dry than most people are aware.