I feel your pain in the confusion trying to sort out between the various companies and even within their product line having been there. I posted the information below as an overview of the Kubota line to a similar question. Perhaps it will help although obviously a generalization.
I will talk in Kubota terms, because I know them better, but there are obviously other good brands.
The BX size Kubota is a SCUT or Sub Compact Utility Tractor. Great mower which can do a lot more than a garden tractor. HP ranges from about 18 - 26 HP. It can have a loader added plus use equipment off the 3pt hitch in the back such a rotary mower commonly called a brush hog for mowing field type grass, a rear finish mower (RFM) or have a mid mount mower (MMM or belly mower). It is small enough to into relatively confined areas such as stall areas maybe. It is limited by weight of the tractor, power of the hydaulics, and perhaps most by clearance under the tractor. But the clearance is only isssue obviously by what you would need to go over.
The B size Kubota is a CUT or Compact. Still a good mower and has the ability to still use a MMM. Also has the 3 pt hitch and power take off on the rear like the BX of course so you can use the mowers on the back as well but can use a little larger. On BX you might want to stick to a 4' rough cut mower but on a B could move up to a 5'. HP ranges from 23 to 32 HP. The tractor is a little heavier, has stronger hydraulics so can therefore handle a little bigger chores. It also has higher ground clearance so less issue with traversing in most areas.
The L size Kubota is a UT. The HP starts about 28 and goes to about 44, although I am not as familiar with it, but that's ball park. Now unless you go to the Grand L, I don't think MMM can be used, so you only have the rear mower options. As with the move from the BX to B, you are moving up in weight of the tractor and power of the hydraulics.With that, the best thing is to go to a local dealer and sit on a few. Once you get a general feel, any dealer should let you test drive them. With minimal explaination, you should be able to at least be able to drive them around the lot. My wife had never been on a tractor when we test drove them, but was comfortable operating a BX and 2 B's as part of the decision. Even if you decide to buy used, this process will help you target what you are looking for.
Like with most things, there are times when it will be too small and sometimes too big. Try to determine what you will be spending most of your time doing try to decide what will work best for those applications and improvise, rent or contract the other work. We bought the B size. It works for us well to do what we do most of the stuff we do and I didn't want to go to an L since I trailer it to our other property, but sometimes I wish for the L, but that is when I am clearing areas of brush. If I was in more of a hurry, I would hire someone with a bulldozer to clear it quick.