Entirely up to you.
The only reason why unit and base sizes matter in Hail Caesar is if it matters to your opponent.
Sounds like you are doing Saxons, Vikings and Normans, so you'll be dealing with all armies for your games?
If that's the case, it's entirely up to you!
It will perhaps be beneficial if all units have the same frontage, but it doesn't really matter if some units are slightly wider or narrower for any reason. If you were playing people with their own armies based in a certain way you might want to confer with them to see how they do it first, but if it's just your armies.....
The depth of the units matters even less.
I'd say do what looks good. I personally think the more ranks the better.
I think it would make sense to have all your saxon, viking and norman heavy infantry units based the same. They'd all pretty much be identical in the game. Three ranks of 10, 30 models per unit sounds cool to me. But then of course if you go for 2 ranks, you can make more units...... Cavalry always look good in 2 ranks. If you have 50 cavalry, that could give you three units of 16 (2x8) with a couple left over?