Our work just added an inbox quota size and I'm curious how close I am to reaching it.
Right click on the very top entry in the folder list named 'Mailbox - Your Name'. Choose 'Properties for Mailbox...', then click on the 'Folder Size' button.
The value for Total Size (including subfolders) is what you want. This is usually displayed in KB. Divide that value by 1024 to get the size in MB.