Tip: Print Sizes Cropping Overlays
Lightroom has had different Cropping Overlays, like a Grid, the rule of thirds, the golden spiral and so on for years now, but in Lightroom 5 Adobe added an important new one that shows you Print Aspect Overlays (like a 5×7 crop, and 2×3, etc.).
To get to this Print Aspect Ratio overlay, get the Crop Tool, then press the letter O until you see the overlay (shown here) with the ratios appearing in a small box beside each one.
Useful little tip, especially for LR beginners like me. Thanks!!