To scan covers I have to leave the lid of the scanner open, I noticed that in daylight I get a better and larger picture than when I scan in the evening by electric light. So in future I shall only scan during the day.
If it's too big, try less than 300dpi ... If you got more than 300dpi set, then it will be quite big. I scanned some covers today, they are 300dpi and will upload. Decent quality can be achieved at lower too