Alright, I'll reveal a bit more about the secret levels now.
The Secret rank is in fact the same size as the others - so, 7 secret levels! Obviously, this means that some ranks have more than one secret level. And yes, this means the total level count is 42 levels (although 14 are repeats of LPIII levels, so only 28 original levels), thus putting it at nearly 2/3rds the size of LPIII itself. o_O
I earlier mentioned that two of them re-use previously-seen concepts that were missing from LPIII. While I'm sure all of you already guessed, these are the PSYCHO 28 / Genius 17 concept (a long level that gives 20 of everything but no builders and requires 100%), and the "No Time To Die" concept. The former I would say is around the same difficulty as the LPDOS/LPII levels of this concept, while the latter is definitely the hardest NTTD yet.
The third is simply a level with the Exhaustion + Lazy combination. It's definitely harder than either of the Timid secret levels, though not overly difficult - probably late Dodgy or early Rough.
The fourth is an entirely original non-gimmick level that makes use of locked exits.
The fifth is a level that uses Surviving Bombers gimmick, though admittedly there's no reason why it couldn't be a non-gimmick level that uses diggers instead of bombers. However, in general it's based on a very unique concept for a Lemmings level.
The sixth is a Frenzy based on a very versatile concept that was used once in Original Lemmings, once in LPDOS, three times in LPII, and twice in LPIII - I won't say what it is but you may be able to guess from these usage counts.
The only thing I'll say about the 7th one is that it's a VGASPEC level with no gimmick. As with the VGASPEC levels in LPIII, it has a custom music track, and is internet-related, though this one is not related to any meme.