Your Notes mailbox is a single file on the server. In the Notes Help documents it is often referred to as a "mail file". All mail folders (Inbox, Sent, Drafts, etc), mail messages, calendar entries, etc. are stored inside this one file.
To check the current size of your mailbox (mail file) open it first, then select Actions - Add-on Features - Show Mailbox Info from the pull-down menus:
Your mail file size will be displayed in a dialogue box that will appear on your screen:
This also tells you how many documents are currently stored in your mailbox (includes messages in all folders, calendar entries, ToDo list entries, etc.), and whether quota has been assigned to your mailbox or not.