If one were to approach challenge dungeon construction through the lens of continuous habit, the question becomes "what's a reasonable self-contained minimal unit that I could make a habit of producing, such that I could produce them in quantity?"
I think for a challenge dungeon, it's probably small dungeon levels, not just individual rooms. A while back I picked up the Mausolean Maze of Mondulac the Mad. It's an interesting product but I never reviewed it properly. It's a collection of stocked, tileable geomorphs with a "hedge maze full of undead" theme. I like its statement of "good vanilla" as an ideal for published products. I think it has a couple problems though. The author adopted the constraint that the level map and key much fit on a single pair of facing pages, which forces small maps and short keys. The most keyed items in any single geomorph is 10, and there are very few (if any) empty rooms. It just feels very dense, and there are only a couple of 'morphs that support 1st-level characters. If you stumble in at 1st and actually do random selection when you move from one to the next as suggested, it's going to be a very rough time.
Tileable / composable small levels as a minimum unit is a pretty promising idea. And in "challenge dungeon" philosophy, each one can challenge one or two tactics. For a "dungeon dimensions" or "mad wizard did it" funhouse dungeon, tiling in euclidean space is also not required for composability. Portals and teleporters solve many problems.
So what is the right size? I think it might be about 15 rooms. Using B/X's or ACKS' stocking tables, this gets you something like 5 empty rooms (one with treasure), 5 monster rooms (likely one lair), 2-3 traps (one with treasure), and 2-3 specials. This seems like about the minimum amount of stuff to get a proper stand-alone "OSR dungeoneering experience". It's enough rooms that it could conceivably be jayquayed, there's enough monsters to maybe pick up some allies against the lair (light faction play), there's likely to be nonzero treasure from the number of empties and traps and maybe the lair. Sufficient empty rooms to rest in, route through, or mistakenly search for traps. If you're tiling these, they could easily each be "a lair and its sphere of influence / territory". Obviously these ratios are a starting point and all parameters are subject to mutation and selection, but it seems about right.
I wonder if such a format is an answer to the Five Room Dungeon meme, which is too small for much jayquaying and usually run very railroady, often quantum-ogre-y, with little interest in player agency...
As for the cadence... I could definitely see doing a 15-room dungeon level per week. Spend a night on the concept and encounter table, a night on the map. The 5 empty rooms are easy, just need a little dressing. That leaves you with 10 rooms to stock in 5 days, so about two rooms a night, some of which are likely to be pretty trivial. And if you actually managed a tiny dungeon per week minus sickness/vacation, you're looking at 50 levels a year. If you take the best 10 of them and glue them together, you've got a 150-room "kilodungeon". And if your players decide to hare off in some other direction, you've got plenty of "b-sides" material ready to go...
One interesting question that perhaps my old prep logs would answer is - if you're running a game concurrently with trying to do this, would one level a week be enough that you could actually "throw away" a good percentage of it? If your players burn through 15 rooms a week, and you prep 15 rooms a week, you aren't accumulating any slack for bad experiments. Maybe this is where the megadungeon comes into play; restocking old areas that players retread frequently might be less work than coming up with new ideas, and the pace of exploration of new areas slows as distances from the entrance increase, so your ability to accumulate a buffer increases over time?
From the logs, it looks like the most new rooms they explored in one session was eight during the first session, and that tapered down a bit for a while as they went back and forth with a lair, and then picked up again and stabilized around five new rooms per session. So maybe 15 rooms per week is actually enough to build up a decent lead. On the other hand, the Dungeon23 approach of one room a day would barely have kept ahead of my old players, provided that the rate of exploration didn't drop off again.
As for actual size of a tile... I think 16x16 is probably plenty. Given 15 rooms or so, if the typical size is 30x30, that uses 135 of the 256 squares in a 16x16 block. So that leaves us with plenty of space for big rooms, long hallways, secrets, etc. And room to deviate up from 15 rooms, I suppose. 12x12 would be adequate if we were willing to commit to minimal space between rooms. 14x14 might be ideal but it's just such a weird gross number, whereas 256 is pleasingly round. If I decide I have the wrong tilesize, oh well. Putting little shim-zones with boring hallways in between tiles seems pretty viable.
I don't know if I want to commit to creating dungeon levels on a cadence, but at the very least embracing the ethos of "don't wait pontificating for perfection, just make stuff and some of it will be good" and fiddling with making levels is probably something I should start doing.