Have you tried lordpe's rebuild function? It may be an easy solution to the problem. If you need to calculate the size of the section manually, simply subtract the section's beginning offset from the last byte in the section +1. If there is a section following this one, you can simply subtract the first section's offset from the from the following section's offset. LordPe is the best tool to do this. Just open the section editor and use the built in hex editor to determine what the last byte is. Once you have calculated the size, just use lordpe to change the value.
If you need a video explanation, the following video for protectionPlus unpacking shows you how to calculate raw size manually:
Code:
http://tuts4you.com/download.php?view.2115