The average size of an ISO would have to be about half a CD.
But really... ISO tracks vary HUGELY, anywhere from a few hundred KB to a full 600MB or so.
Anyway, you might get incomplete downloads of rar/zip/ace archives but you'd know by the time you extract them. It would be very rare to actually end up with an incomplete ISO inside a complete archive... I haven't seen THAT happen yet.
Even so, if you're not sure about a given ISO, just run it through ISOBuster and attempt to extract all of the files and folders in it to some temporary directory. If the extraction finishes without errors, you know the ISO is complete.
(Actually, you really only need to test/extract the files near the end of the ISO track, i.e. those with the highest LBA values. If you don't understand what that means, nevermind.)