I used to use one which used a mid-sized drum mounted into the middle of a 1/4" plate hearth which is 2' x 3' and legs welded on. Also scrounged up enough old bed rails from the dump to weld a nice lip around the outside edge of the hearth to keep coal, etc from falling off. Drum forges are quick and fairly easy to build, almost made for the pipe-flange that connects to the bottom for your air, but....
The cons are that you have the lip of the drum sticking up above your hearth which is a pain, not only for your stock being placed in to be heated, but also getting enough coal in. To slot the sides of the drum to allow your stock to sit flatter in the pot requires a lot of grinder blades or someone with an air-arc setup to cut it as it's a bugger. I finally got sick of the drum due to the limitations, bought more plate and welded up a new pot. Welded it flush with the hole in the hearth so the top is nice and smoothe with no obstructions to get in the way and I couldn't be happier. Plenty of coal laying about the hearth to pull into the pot as needed and no aggravating lip to keep the stock out of the fire. I didn't realize that the bigger stock would need so much room til I tried the drum and that's another I switched. Save yourself some frustration later on and get the stock to build a firepot or just buy a pre-made pot. Mount it flush to the bottom of your hearth and I bet you'll be very glad you did
Good luck with your forge