Or maybe I can somehow optimize its size?
4K is fine, 8K is over the top 
This guy wrote a material that shows you how much resolution you need as you move around the level
Use Above material, or just get idea from his video.
However there is one more aspect to using megascans huge textures. It is how much video ram you have on your dev pc. You can always use mip maps to lower resolution of packaged textures. But if source texture is 4k or 8k and you need to package and resize hundreds of them your pc may derp to desktop or worse do happy restart.
So as long as you have some 4090ti and it is not smelling funny you can use quixel textures in 4k. Then package project to something sane like 2k.
Also remember quixel sometimes uses 4 4k textures (that makes one 8k). Best is if you make layered material that does not use quixel skin textures, but instead masks and blended texture.
Final Fantasy 7 Rebirth is like 90% Megascans