You can see the sizes when setting up pressure plates, or atleast a few examples.
You could also use a series of pressure plates set to trigger on different sizes to determine the size of a creature. What I mean is you'd make a long hallway with alternating pressure plates and floor hatches. The pressure plates would be arranged such that the one at one end would accept only size 2 creatures, while moving towards the other end the size of creature triggering the plates would get increasingly large until the last plate was triggered only by size 20 creatures. Each plate would be linked to the hatch directly before it and directly after it, so that when a creature triggered one it would get stuck on the triggering plate, thus showing specifically its size.
Sort of like a coin-counting machine, actually, if you understand the principles behind that... Or maybe I'm just insane in thinking that this is like a coin-counting machine
I have seen indications of a creature's size not being changed by Dwarf Companion when changing a creature into another creature. Even after a game reload.
I have heard that creature sizes are determined when they spawn. If so, you could do some extensive workarounds involving babies and size modding. That is, get a bunch of female domestic creatures of some sort, give them the [CHILD:100] tag and the desired size, let them breed until you have a satisfactory number of particularly-sized babies (since they are born/spawned when the [SIZE] tag is at the desired number, the young will be the right size even if the mother isn't), then recollect the mothers, remove the [CHILD] tag to allow the young to grow up quickly, morph the young, then repeat with a differently-sized creature.
...Of course, now we're getting into truly ludicrous tactics that are probably vastly more trouble than they're worth, what with the arena feature that is supposedly going to be out with the new version which is due by next year