i make maps bigger than any others that i personally see, so i doubt that is the issue. however, there is a minimum height, and if you place a map too low, it is basically a un-fixable kill zone when you go down there (like my asteroid base where you will die before you hit the ground if you are jumping into the black abyss of the reactor room) this can be fixed by keeping your map around the blue grid.
failing to build all before playing a map can also cause the dying issue
other than that, id check to make sure everything copied over. sometimes copy and pasting from SP to a new map does not go so well and pieces get lost along the way, be sure that you have all brushes where they need to be and your player start is inside one of them
you might want to make sure that there are no triggers/events/ai stuff left over from the SP (ie be sure that the only thing in there is brushes and static mesh)