You could create seed files with embeded metadata on playarea size and not allow loading seeds that were created in larger playspaces.
This is so obvious and elegant solution. I wanted to add size to the seed thingy separately but making it part of it, makes more sense. Thanks :)
But still, it should be allowed to play seed with different size too, as the general layout of the level will be the same, just the details will be different. But taht would be an option and if it confuses people, I should drop it.