Unit size suggestions?

Just from what I’ve seen, I’d bet wider frontages are going to be pretty common. You may still want to field Skaven in 40+ model units, maybe 8x5 since it seems likely they’ll have Horde and get up to 3 for a rank bonus.

Looks like the max rank bonus for infantry is +2. The unit bases are 25mm at minimum, so 10 wide could be hard to handle.
In order to have a valid formation, your unit needs to be more models wide than deep.
Maybe 8x3 or 8x4?
Hard to say without having a few games.

I plan on playing High Elves and I think I'm going 6x5 for Spearmen, and then 8x3 for my elites like Swordmasters and White Lions, still up in the air about Phoenix Guard, they're still tankier than the other elites, but also get to fight in an extra rank with their special halberds. And they count as Heavy Infantry which as I understand is a 4 model minimum for a complete rank.

Width has its merrit. But there are many tactics besides that, like MSU. 20 wide units sounds nice but they are 50 cm wide on the board, so easy to negate with some simple chaff.

I was thinking running orcs at 24, and keep the option of running them 6 wide, but also being able to setup as 8 wide. I can see MSU work but prefer Orc by numbers.

A 20 wide unit of Orcs is 2" across. That's a third of the width of the battlefield!
Besides being almost impossible to manoeuvre it'll be a target for every bit of shooting available.

In the one game I've played I had (Empire) units three or four ranks deep and six or eight wide.
You need bodies to soak up the attacks coming at you as high initiative things may mean you don't get to fight back.

You will, I think, win games by either wiping out enemy units on the charge or by holding them and then charging the flanks.
FBIGO and Give Ground make it really hard to break units. So if you don't kill *everything* on the charge a good player will have something hitting you next turn.

Units trying to wipe you out in a turn or two want to kick out a lot of attacks, so will aim to go wide.
But units trying to hold you up will counter that by going narrow (as only the ones in direct B2B get multiple attacks).

So I think you want spears in blocks of six by four or five, or even five by five square, for those defensive, "anvil" units.
You need them four or more deep as you'll probably lose at least one whole rank in the first round.

If anvil units are five or six wide you probably want your hammer units to be seven or eight wide - if you go any wider you're paying the premium for the additional attacks you won't get to use.

But I have played only one game and am rubbish at Warhammer, so all of this may be complete nonsense.