There are lots of threads here that show how to make one yourself using square
water tanks in frames, or round water tanks, rectangular storage boxes, making sure they are on a slant so that they drain well. It isn't necessary to buy a kit or a pre-existing one.
I drew a diagram at this site that has the inlet pipe at 3" (usually the size coming from the house, or 4"), and the outlet pipe at 2", but I wouldn't go smaller than that. The worm castings will wash out and down the line into some kind of
perennial landscaping or big reed boxes. But worm casting are such valuable things, they
should be used.
One of the
online diagrams shows about a 1" or smaller outlet, and I would not recommend that. Worms can drown, and don't need to have the water stay in the tank. In fact, it should just pass through.
Worms can freeze or drown or bake, so it needs to be insulated or buried safely with an insulated removable lid. You want a couple of 45 Degree outlets that will let you run a snake down the outlet line just in case. Occasionally the rock gabion in my diagram, that surrounds the outlet pipe that is in thick chickenwire, can be lifted out, rinsed off, and put back if necessary. But I haven't had to do that in the one I've had for 2 years.
To size it, I would estimate 50 gallons per person. The plastic water tanks in a frame are not that big for a 200 gallon tank, and can be put in place easily.
I have two of these worm boxes, and I wouldn't do it any other way.
They are amazingly effective and do not smell.
But if you are living somewhere and the neighbors get wind of it, or the
city or county gets wind of it, they might really cause you problems. Don't entirely undo whatever septic situation you have just in case, because redoing one of those with permits is crazy expensive.