From the few times I've used hand-cranked post drills, I went with a 1/4" diameter pilot drill. Pushing a properly sharpened 1/4" drill thru A-36 structural steel with the "peck feed" on the post drill engaged was not hard work at all. I then stepped up, 3/8", then 1/2", and then 5/8".
I got one post drill from the son of its original owner/purchaser. A fellow named Hank was retiring from a mechanic's job on the NYC Watersheds in about 1986 or thereabouts. Hank called me to ask if I would be interested in some stuff he had left from his father's blacksmith shop. Hanks's father had a blacksmith shop in Croton Falls, NY. It closed in 1952 when Hank's father died. According to Hank the shop never had electricity. Hank sold me a Canedy-Otto farrier's forge which his father had mounted on a Model A truck chassis. With it, he sold me a Peter Wright anvil and a Champion Blower and Forge post drill, and some other odds and ends loaded into wood boxes from welding flux and toe calks (for draft horse shoes). Hank told me that as a kid, he HATED that Champion post drill. It seemed his father would take jobs to build ornamental "iron" fences and gates. Hank would drill endless holes in the fence top and bottom "rails" (flat stock) for the tennons on the "pickets" to enter. According to Hank, if the chips were not heaping on the floor and coming off hot, his father would be hollering.
After Hank's father died, Hank cleaned out the smith shop, and sold the property. He mounted the post drill on a wall in his home's garage and put a large sheet-metal vee pulley on in place of the flywheel. Using a 1/4 HP electric motor salvaged from a washing machine, Hank said that drill had no problem pushing good sized drills thru steel, albeit slowly.
When I got the tools from Hank, I noticed that Hanks Father's name, and simply "Croton Falls, NY" was painted in old-time handwriting with some kind of paint marker on the back of the mounting plank furnished as part of the post drill. At a yard sale, I found a fellow who had a load of curved-spoke flywheels with the old square-headed set screws. Apparently new-old-stock from some mill supply or similar. I bought a few, and one is going onto the Champion post drill. I have another Champion post drill my wife bought me, another yard sale find. She figured it was a blacksmith tool, and the fellow selling was unsure what he had, so for a few bucks he put it in my wife's Blazer. Hank's father's post drill has the "tire hanger" (for drilling bolt holes in steel wagon wheel tires), rather than the table. The drill my wife bought me has both the tire hanger and the table.
Truth to tell, I use neither of the post drills. If I have heavy drilling over 1/2" in diameter, I do it in my Cincinnati-Bickford "camelback" drill, a machine old enough to be in keeping with a blacksmith shop. I am hoping to build a 12' x 20' shed (finally) and put my forge, anvil, leg vise, swage block, and smithing tools out there. When I get that shed built for my smithy, I will mount the post drills. I have a 30" x 40" Champion forge hearth with their "whirlwind" firepot, and the 400 series hand cranked blower. I've toyed with the idea of leaving the smithy without electric power, just relying on a Coleman lantern or two if I run into darkness, and relying on basic smithing methods for the most part.