Here's the picture (sorry about the poor quality----it was cropped from a larger picture of not great quality) of the small retort I built years ago. It's real simple-----the body diameter is about 4", with a flanged top, maybe 5" diameter. A larger size isn't real important unless you have a lot of material to process on a repeat basis. You can process a serious amount of material with one that size.
I don't recall the capacity, but the pot holds a couple of cups, maybe three. So you get an idea of size, the condensing tube is ½" diameter stainless, and the cooling tube that you see on the condensing tube was made from 2" stainless tubing. There's a couple thin stainless plates on each end that are silver soldered to the tubes, plus two fittings for water inlet and egress. I used 3/8" vinyl hose for the water supply. and a small pump to re-circulate water from a bucket. You could use tap water and run it to waste, but you'd waste a lot of water. Water should be introduced to the bottom fitting and allowed to overflow from the top fitting, assuring that the cooling chamber is always filled with water. You'll notice a fabric boot on the end of the condensing tube. You use that, slightly submerged in water, so if under some strange circumstance the retort pulls a vacuum, it allows air to be pulled, plus by keeping it only slightly submerged in water, it runs out of water before enough enters the tube to fill the retort.
The retort is held together by a half dozen C type clamps, made from stainless, drilled and tapped ¼"-20, with stainless hex head cap screws. It's important that an anti-seize agent be used to prevent the screws from galling to the clamps.
Do not make a retort from steel-----it will oxidize rapidly and render itself useless in short order. Ductile or gray iron will hold up much better, with ductile being the better choice because it has greater ductility and can be welded easier. It's important to have a metal lathe and mill at your disposal if you intend to build one. I can also answer any questions you may have. I've held back on details so it won't bore you to tears.
Harold