Do you have enough experience and determination to create a mmorpg?
You can make a really big world with any terrain editor. I mean you will connect terrains with loading screens like any other professional mmorpgs in the commercial market. You can make a huge world in one piece. Not with Blitz3D anyway.
Wings dude have a "void rpg online" called mmorpg, he can help you better as I haven't tried any mmorpg.
http://www.voidrpg.com/