I've actually wintered in as few as two mediums and they did fine, I guess that could've been considered a NUC, but I didn't know that at the time. I've been downsizing everything to mediums since reading MB a few years ago and am trying to build more Long hives and/or maybe convert further some of my mediums down to 8 frame. I've got a pretty bad back and lighter really "IS" better!!!!
When I got into this (after a 30+year break) like most folks I read a "little" bit and then bought a couple Lang kits and "THEN" started "really" reading and have been paying for that mistake ever since.
As I said about clusters in my current hives, can't see the one in Long Hive, but I know its there, can see portions of others (besides my quiet one) and they're all "near the top" but not right on at it, more like a half a frame or better below it seems, surrounded by honey. This is all based on what I can see you know, as they are still covered with dry sugar, so I suppose they then "could" be right there. Anyway my quiet one, I couldn' see at all, but could hear, perhaps (I'm hoping) they're just tiny and hanging on the bottom of the sugar I gave them in November chowing it down.
This certainly isn't meant as a dispute, from what I can tell my bees clusters are simply not at the absolute frame top, as some have suggested they should/would be, near but not quite there and I don't know where for sure the Long Hive cluster is on the frames, just a general location (one of the issues "if" you're gonna go into your hives in winter :-\.
Its the first time I've ever looked inside during such weather (29F but no wind) and I don't believe I'll ever do it again, been reading too many southern beek posts, I guess :-D
One of The best tips I ever learned was sqeezing them down as winter aproaches, an empty frame or super is just unacceptable so I remove them. That way I can start fall with say five or six boxes and end up with 3 or even two, not including a feeder box, for winter.
I'll likely keep everything in mediums and keep making boxes to fit them, (although my Long hive is filled with cut off deep frames with just top bar and about half the side, the bees keep the bottom open on their own) with some minor adjusting now and then, always a ;)justing. All Mediums are working for me and its a luxury to have all interchangable frames and boxes.