Couldn't make it work with K2 Tools, but using HD-Custom CSS with that code
the mainbody is now full-width. Now, can I expand the mainbody to fill the window vertically?
The calendar is now an iframe of a Google Calendar page -- because the GCalendar content was picking up some odd formatting when the custom CSS module was active -- but I don't think that changes the question about making the mainbody full-height (within margins).
Setting the Calendar menu's "Scroll bars parameters: Height" in pixels works just fine, but setting it to 100% (or to any percentage value) just reduces it to what appears to be the absolute minimum height for a Google Calendar page, 152 pixels. If I add "height: 100%;" to that custom CSS, the calendar will display for about one second at the height corresponding to the "Scroll bars parameters: Height" setting and then collapse to 4 pixels high.