Small splits are really nice to get a cold stove quickly up to temperature. A hot burning stove is a clean burning stove and a clean chimney. Big splits are less work and burn fine once the stove is hot.
I split just small enough so Mrs Papi can load the stove if she needs to most of the time. Exception would be red oak, since it takes forever to dry, and I don't have that sort of time burning 15 cord yearly. Still working on year two of the three year plan here. Just being a year ahead has made a world of difference in our burning.
If you have wood that you need to season quickly, small splits help. Otoh, if you have time to wait, the large splits are great once you have a good fire going or as the bottom layer of a top down fire. Bigger burns slower... Less total surface area compare to many smaller splits.
Have you measured it or is that what it says in the manual? It's something of a time waster from a cutting standpoint but having short rounds will allow you to load your rather shallow firebox north south, something you will want to do for long burning top down fires
17" would be measured and the max I can put in and get it to lay down. 12" is from memory. If I remember correctly it measured about 11 7/8". The manual suggests loading in the 12" direction. I have not burnt very much at all in that orientation. I do have a skid in one of the out buildings of shorts cut in the fall 2016 and 2 skids in the barn of shorts cut Jan-Feb. of this year. Waiting on it to get very cold before I bring them up. If the shorts produce markingly more heat I will start cutting at 11". As far as small splits. I am talking perhaps a 12" rounds split into eights. Lately I have been quartering them.
It's not so much more heat from the top down, it's longer heat. The amount of heat is tied to how much is burning at one time, how hot the flame is, and how quickly you are exchanging the air in the fire box.
Regular sized (relative) splits - 85%. "All-nighters" - 15%. All-nighters are typically 2 or 3x size of typical split.
Since I have an outdoor boiler, my split size is regulated by how fast the wood dries. I split ash and soft maple into larger pieces than I would for beech or sugar maple, oak gets split very small when it's in my stacks.
