Hi Richard,
How to know overall size:
Your total size is in the very upper right corner, in a lavender box. This is also noted when using the link "User -> Preferences" from the top menu bar.
The size of an individual history is noted in the right history panel, located underneath the "Options" pull-down menu.
How to delete permanently:
Please see the wiki about Managing Datasets at: http://galaxyproject.org/wiki/Learn/Managing Datasets#Actions Specifically, the section about Delete vs Delete Permanently, although tips for moving data around are throughout the document.
In short: if you just delete, the data is recoverable (by you), and it still counts as part of your quota. If you delete permanently, then it is not recoverable (by anyone, including admins), and is removed from your quota. It is probably worth the time to copy/export/save back anything to may miss when first testing out the "delete permanently" functions for individual datasets and complete histories.
Hopefully this helps. I am posting back to galaxy-user@bx.psu.edu for others that may need similar help. All of this is in the wiki, but more detail (graphics, etc.) is planned for in the near-term to clarify the functions further.
Best,
Jen Galaxy team