Grid overlays aren’t limited to cropping anymore! In Lightroom 5 , you can choose to display a grid in the Library (while in Loupe view), the Develop module and during Tethered Capture by selecting View > Loupe Overlay > Grid. Note: by default the Grid menu option displays a dash (not a check) before its name because the overlay is not visible by default. To toggle the display of the overlay, select Grid from the menu (or use Command + Option + O (Mac) | Control + Alt + O (Win).
While the grid is visible, Command (Mac) | Control (Win) displays options for Size and Opacity. Click-drag left/right on Size to decrease/increase the grid size. Likewise, Click -drag left/right on Opacity to decrease/increase the grid opacity.