https://forum.axishistory.com/viewtopic.php?t=61650
"In the civilian crematoria there was an introduction opening (muffle) for burning of the corpse, in rare cases two. In the crematoria for the concentration camps there were three introduction openings. The size of the introduction openings at the crematoria for concentration camps is smaller - 70 x 70 cm -, the length is two meters, as opposed to two meters and thirty centimeters in civilian crematoria.
Instead of a rolling car on rails, on which the corpse inside a coffin goes into the introduction opening, in the crematoria for the concentration camps the corpse is inserted on a hand-carried litter without a coffin."
70cm is 2 feet 3 inches. Regrading multiple corpses;
"I mentioned to Sander as an example that in my presence two corpses had been introduced into an introduction opening/muffle instead of a single one"
"In the common crematoria there was only one huge opening (muffle), into which a coffin with one corpse could be introduced without difficulty. In the crematoria of the concentration camps the ovens had not one but three openings"
The openings into each oven were smaller and there were more of them.