On my test machine, I have the Number of content revisions set to 3 and the size of the Content folder is approx 2GB. On another case, the number was set to 120 revisions and the size of the Content folder was around 110GB.
So the size of one content revision would be approximately around 600-800 MBs. Considering that, I guess 6.5GB would be a normal size for 10 content revisions.
You can leave the value to 10 if your disk capacity allows. The impact would be that if a SEP client which has not connected to the SEPM in a while (Eg: User is on vacation, etc) is outdated beyond the number of content revisions mentioned, it would prompt the SEPM to download the Full Definition file which would cost bandwidth.