I've got a 6x4 trailer too, from CLH, that easily accommodated 5 finished pigs last Xmas, and when I got my 5 shetland sheep there was loads of room, well enough for a couple more sheep, or if they were shorn 10 would have fitted in ok!
Didn't know there were specific sizes though...
Having recently been told off at the abattoir by a woman with a clip board for not having loading gates on, I took the trailer back to the manufacturer and asked them to make a couple of modifications - I explained they weren't safe to leave on. So the loading gate hinges are now drilled and split pins hold the gates in place so snouts can't hoik them off during the journey and break a leg!
I also have a mesh partition that makes a second separate compartment. I asked for a set of hinges to be added at the back of the trailer to hang the partition from so that the ramp could be lowered, the loading gates opened and the animals would still be restrained. Hinges again drilled with split pins. The ramp had a nasty habit of pinging open when we went over a bump, ok going to the tip but not good on the M4 going to the abattoir!