I guess you are using basic World Editor, if you want to get rid of limits you'll have to use some editor extensions(JNGP or WorldEdit Unlimited).
I suggest JassNewGenPack:
http://www.hiveworkshop.com/forums/tools-560/jassnewgenpack-5d-227445/
And click "Enable No Limits" in the Grimoire Tab.
it also allows you to make up to 480x480 map, makes backups, so you won't lose your work and a lot more. Don't worry about the anti-virus/chrome warnings, it is labeled as virus because it "infects" the original editor(or something like that).