Sort the Recycle Bin by Item Type, then select all the files, do not select any directories. At the bottom you will be able to see the size of all the files if you show detailed information.
An alternative way is to select all the hidden system folders called $Recycle.bin in the System Root and viewing the details or properties of your selection, but above method should just do...
DIR /S %SYSTEMDRIVE%\$RECYCLE.BIN | FINDSTR /C:File(s)
See the last entry of above command to get the size of the recycle bin from a command prompt.
I wonder why it's important to know the size of the recycle bin though, when you want to know how much space you earn it's better to use Disk Cleanup or a similar tool, but for just getting rid of your deleted items it is not necessary. I think they left that feature out because it would have to look at different recycle bins, but indeed, with an extra effort it wouldn't be hard to implement...
