IBM PC/XT Enter keys were 1u. The key was 3u tall, but the raised striking pad was only 1u. In fact, all of the striking pads on the XT were 1u, even when the bases were larger.
I'm running an IBM AT keyboard now. It has evolved over the years. I swapped the Big-Ass-Enter for an ISO Enter, didn't like it, and swapped on an ANSI, which I liked a lot better. Later I decided I wanted to use the left-hand switch hidden under the ANSI Enter, so I split it into a 1u on the left and a 1.25u Enter on the right. Which I didn't expect I'd like, but it turned out to work just fine.
Right now I have 91 keys, with all of them split except the bottom-right-corner, which used to be a 2u-high "+" and is now a 2u-high Enter, same as the usual 101+key layout. I like karate-chopping the right side Enter key.
The only split that was troublesome was splitting the left Shift from a 2.25u to a 1.25u and a 1u. It took over six months before I got used to that, as I had a habit of striking the left side of the key for some shifts and the right side for other, plus for some reason I often used my thumb instead of my little finger. Unlearning all that took a while.
I kept the 1u Backspace key; corner keys are prime real estate, and I never had any trouble hitting it. Though I admit when I'm using a keyboard with a 2u Backspace, I probably use the left side of the key more than the right.
Do *not* visit forum.colemak.com if you value your sanity. You'll start thinking "gee, it would be so much more convenient if this key were moved over there, and..." ...and you'll go off down that rabbit hole, possibly never to return.