How can I tell the total amount of space used by a folder (including sub folders)?

A. There are two ways of doing this (there are more!), one using explorer and one from the command line. Using Explorer

From the command line you can just use the dir command with /s qualifier which also lists all sub-directories, e.g.
dir/s d:savilltechhomepage
would list all files/folders in the savilltechhomepage directory and at the end the total size.