Post-migration, the Greenby.com portal gives a very easy way to see what the overall email space usage in KB/MB and as a graphical and numeric percentage, and what they are for each individual mailbox. You can also set a space limit for each mailbox (which is a hard limit).
The usage can be seen from the Welcome page by simply clicking on the Email tab or menu item, and a list of mailboxes is displayed with space details, with the overall figures at the bottom. To change a mailbox limit, click on the 3 dots to the right, then select Edit Storage Space.
I think these figures change in real time or very soon after.
To find usage for a folder within a mailbox, webmail should be used as described by @Townman above.
