First, please read this post and help us to help you by giving us a LOT more information about your computer, the exact version and variant of MEP you have, the type of disc you are attempting to burn your video to, etc., etc.
Also please tell us the exact wording of the error message you are seeing, because there are at least two possible "sources" for that message:
The disc you are trying to burn to does not have sufficient space
The Export process requires your computer hard drive to have sufficient free space to actually create the files to be burned to the disc and your hard drive simply does not have enough!
However, please answer ALL the questions and requests for additional information in my post above . We would like to try and help you but without a lot more information from you we really cannot.