To select an exact area in a map, select the 'Edit→Select area…' menu item. This will open the 'Select map area' window, shown below:
In this window, you may set the exact coordinates to be selected.
To select all pixels in the active map layer, select the 'Edit→Select all' menu option, or press CTRL+A.
To clear the selected area, select the 'Edit→De-select' menu option, or press CTRL+D.