(off topic) File compression

This is off-topic, but I was wondering if anyone knows how to
determine the uncompressed size of a compressed file?

We have several compressed folders on our network that display their
size. They are compressed through folder properties and checking
compress. I assume the size they display is their compressed size.
How can I find out the uncompressed size? Is this info stored
anywhere? If not, is there some sort of ratio I can use to
estimate. Thanks so much for any help on this!