1024512, 20481024 etc. in certain cases this could be advantageous and I’ve read somewhere that they are being used.
Question: is there any catch? a performance penalty for example or it’s well supported in UE4.
I haven’t actually tried to UV unwrap to a rectangular texturte, i wonder if it’s requires special workflow. (I’m using Blender.)
mixing texture dimensions is fine as long as they are still power ot 2.
As for Uving, you don’t do anything ‘differently’ really when you have a rectangular image. It always gets squashed to fit in the 0-1 UV range. As long as your proportions are the same between UV and model, you will be all good.
This is the concept of things with Maya/Blender…and I would assume this is the case for Unreal as well.
Yeah I’m doing this at present with some Environment Assets that are a bit too big for just one 2K Texture, so I’m mapping them to a 2048 x 4096 (Two 2Ks side by side as a rectangle) in a single texture. Hope it all works fine with MipMaps and stuff! 
I’m using Blender too as always!