I have a idea, what seems nonsensical at first, but it could be good in practice. Normall 1-battalion divisions are cheesy reserves - you make them to convert them into something useful, or use them in situations what don't require actually fighting someone - like charging deep into territory or executing Order 66. But there is an idea of actually combat-worthy tiny divisions:

You would take 1 battalion and as much support companies as possible (at maximum of 5), basically making divisions composed mostly of supports; this is because you can't make divisions made entirely of support companies (if i am wrong in that instance, you can try that too). Because support companies don't take front width, that translates into more bonuses per width; of course these tiny divisions must group into "piles", with each pile being same width as a sane man's division (for example, if we want our pile of microdivisions to have width 40, and each of them have combat width of 2, like Infantry, microdivisions must group in swarm of 20 - and that is equivalent of 1 division, now if country would normall have 100 divisions, it would translate into 2000 microdivisions).

Obvious minuses is what they won't fit into any commander's limits, and it would be extra hard to control them, both macro and micro (you must make sure they don't separate when that is not needed and don't overcrowd, and also disperse and concentrate them in right moments). But most "meta" ideas from HoI4 would look like nonsense when taken out of context and put IRL - so, this 1 battalion + support divisions should be analyzed (as no one has tried that idea before).

How would they fare against "normal" divisions, given if we take both of them of same combat width, or of the same cost, or training time?

Also, would divisions made just of 1 battalion work in battle (for example, 10 divisions with 1 infantry battalion in each versus 1 normal 20 width infantry division)? I don't think so, if they are taken individually they loose, but what if each group of tiny divisions will go in pack, results could be diffrent?

The logic behind it what divisions gets debuffs if it is overly large, so if we extrapolate that to other end of scale, group of small divisions is stronger (?) than a normal-sized division of same composition. Also they would fit pretty much into any terrain's width with no problems, and also increase the technical amount of used divisions, what could affect things like other country getting scared by sheer number of them, or amount of volunteers what can be sent to you.

Also, is it a good idea to go in opposite direction - make huge divisions of 25 battalions and 5 supports (at cost of having less divisions overall)? They would have combat width issues though, is it's also probably bad.

Both "big divisions" and "tiny divisions" actual combat strength better be seen in normal gameplay or multiplayer; in speedrun the 1 battalion only division's role is clearly capturing unoccupied lands, not getting into actual combat. Meanwhile the "25+5" and "1+5" divs are made for actually fighting enemy. 1 battalion division and "1+5" division are different things for different purposes.