Can I select (highlight) the entire spreadsheet?

Yes. Following standard spreadsheet conventions, click in the upper-left corner of the spreadsheet (in the Info box). This shortcut is useful, for example, when you intend to copy the entire file to the Clipboard or reformat the entire spreadsheet. You can also press CTRL+A or select Select All from the spreadsheet Edit tab or menu or the shortcut menu (accessed by right-clicking in the Info Box).

Note that by clicking twice in the Info box, you deselect (i.e., select and then deselect) the entire spreadsheet.