When you start Nero, set your preferences, etc and choose Recorder -> Burn Image. Make a note of the info tab. If it says mode 1 with a size of higher than 700MB, then something is wrong. Check the Foreign Image tab, and see what it says. If it's Mode1 or Mode 2/XA 2352, the ISO wasnt created properly, and the "source" screwed up. If it's Mode 1 2048, it really is too big... Again, in this case, the source screwed up. However, speculation must be shoved aside. It's the end result that matters. Just to be sure, Select DAO/96 as the burning mode, and select a simulation run, and uncheck write. This will be the test of whether or not that is a bad ISO. Perhaps the best thing to do is "find another source".
If a data track is over 704MB, it cannot be written to on a high density (700) disc. To make an ISO of anything more than that is stupid. Especially when the said ISO is less than a gig, which wont work for a DVD.
EDIT: I am backwards.