I need suggestion on what format use for my mod, esp or esm. Reading around i see esm give some problems with world cells editing on the world spaces that ships with the game, but in my mod I'm gonna put also new world spaces completly separate from the main land.

Now I read that esp have a limit on the size and I don't know how big is gonna be after completition, so what would you suggest to use, a combination of both? Like one main esm and an esp with land info, or I can just keep only the esp?

if you are new to this sort of mod, you want to start with .esp, and when you have a bunch of content in your mod, at that point you can really start to decide if an .esm format is right for what you have.