"What are the dimensions of this world?"
There are no dimensions in Minetest.
"I seem to have reached the end of it, so that one without fly privileges enabled would fall off the edge of the world."
No, you can't. The Minetest's collision algorithm forbid that. You can't pass through the edge.
my world must be corrupted then, because my map just drops off and one could fly off and down the edge. how do i fix it?