Select "Show Grid Overlay" from the view menu to display a grid on images viewed in main and thumbnail views. The color and spacing of the grid lines can be specified using the dialog below which can be displayed by selecting "Grid settings..." from the View menu:
Select the show print area checkbox to show the extent of a particular print size. For example most digital SLRs shoot images with an aspect ratio of 3:1 which fits perfectly on a 6"x4" print but needs to be cropped if it is printed on 10"x8" paper. You can display extent of a 10"x8" print by setting the print aspect ratio to 10 x 8.