Best approach/Practice to design Massive maps

I’m talking about designing massive persistent world like MMO. I don’t now much about level design I’m more into coding side.
What I found is “level streaming” is the way to go, so is there a better way?
If no, then what is the pros/cons of having too large(obviously memory cost will be higher but anything else) or too small landscape
and what is a good map size should I create for each level to have stable maps, i’m using the engine landscape tool to build it.
Also is there a good book on that topic