Folder Size Custom Script

I am about to do upgrades and need to know how to check specific folder sizes. The custom script to check folder size suggests using <Documents>, <Music>, etc as a script argument. However, when I type in <Documents> as it suggests, the error log returns:

Get-ChildItem : Cannot find path 'C:\Program Files (x86)\DesktopCentral_Agent\C
omputer\Startup\1809\<Documents>' because it does not exist.
At C:\Program Files (x86)\DesktopCentral_Agent\Computer\Startup\1809\FolderSize
.ps1:13 char:27

It is looking in the install directory it appears.

I need the folder size of the currently logged in user. Are there %currentuser% variables or the like to use where the script will know to look in the right path?

Thank you