Depends on the file system, I know for a fact that ZFS supports ssd caches (in the form of l2arc and slog) and I believe that lvm does something similar (although I’ve never used it).
As for the size, it really depends how big the downloads are if you’re not downloading the biggest 4k movies in existence then you should be fine with something reasonably small like a 250 or 500gb ssd (although I’d always recommend higher because of durability and speed)
BombOmOm