One of the nifty new features that snuck quietly into Lightroom 3 and is now a staple of later versions of Lightroom was the ability to resize to a specific target size measured in terms of megapixels (abbreviated as MP), the measurement system used by camera manufacturers that combines width and height. While this is a niche feature, one niche it comes in very handy for is stock photography.
Royalty free stock photos are often priced according to the image size. Larger versions cost more than smaller versions. Different stock photo agencies determine the size thresholds in different ways. Some, like iStockPhoto, use pixel dimensions in width by height. Alamy uses a derivative of filesize. But others, like Dreamstime and Shutterstock, use megapixels. While it's possible to approximate the effect by using the traditional width x height measurements, the ability to specify a megapixel count adds much more precision and reliability, especially for images that fall outside of standard aspect ratios.
The feature is really easy to use. In the Export dialog, choose the "Image Sizing" option box and choose "Megapixels" from the drop-down list next to "Resize to Fit" (you have to have the box checked or the drop-down list will be grayed out).
This is also handy for those photographers who might want to control which size options their images are offered in at a particular agency, for whatever reason. Some photographers, for instance, only offer minimum sized images at some agencies where larger sizes don't mean larger payments (such as Shutterstock and larger versions at others, where price increases with size (like Dreamstime).
And while most stock agencies discourage or ban upsizing (Alamy is a notable exception), downsizing can also sometimes be a handy option for salvaging an image with borderline quality issues. For instance, an image shot at a high ISO at native 12MP might have problems passing quality control due to noise issues, but by downsizing it to closer to the minimum accepted size such as, say, 4MP in Shutterstock's case, it might have much more chance of getting accepted with even the strictest reviewing process. The same thing goes for scans of film negatives.
Bug in Lightroom 5.3 Release Candidate
Lightroom 5.3 Release Candidate introduced a bug related to the "don't enlarge" button. The bug remains in the finished version of 5.3. If you leave the "don't enlarge" option unchecked, it should work normally. But if you check the "don't enlarge" option, it stops the resizing to megapixels from working correctly. You can find more information here.