folder sizes aren't specifically stored anywhere as a variable, they are calculated when you right click -> properties on the 'c:\users' profile folder. Any location or subset data you need will be found in the registry under this key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
If you need to collate a list of profile sizes, it would be best to query 'c:\users'. powershell would be my first go-to for this. Note also that the 'c:\users' location will only contain the complete profile if roaming profiles is not enabled and that you are not using a folder redirection policy, otherwise you would need to query the location they are stored in.
If you are interested in the powershell method, please see below link. This will detail how to use the folder-size module then you will just need to add in the folder name for identification. An off the shelf product which will be able to grab this for you is called 'spacesniffer' - run this againt your c:\users and it will provide you with a graph showing different shape sizes for each profile and you can even export that into a file if need be.
edit: forgot link https://www.gngrninja.com/script-ninja/2016/5/24/powershell-calculating-folder-sizes
spacesniffer: http://www.uderzo.it/main_products/space_sniffer/