Size is the size of the QVW that was scanned as part of the "Documents | List of Files Paths to Scan". 750 means the QVW is 750 MB. (Note that Windows Explorer might show a slightly different file size.)
Sessions: With no other selections made, Sessions denotes the total number of sessions opened for that document on server - for all the history of server Sessions.log files that were scanned. View more time-specific session data for that app on the Sessions tab (of GovDB 1.1).
For other specific questions, you might consider doing a text search on the online Help documentation for the Dashboard, HERE (for GovDB 1.1).
Like any QVW app, the Governance Dashboard is not real-time but requires updating (reload) either manually or via Publisher. Typical scenario is for a client to install and configure the Dashboard and then setup a doc task in Publisher to update the app daily (or more- or less-often as deemed useful).