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Lightroom's crop tool one of those things that people seem to either just get intuitively or not at all. Matt Kloskowski has a great video tutorial on the crop tool in the Lightroom resource center, but I wanted to try and clear up the most common question I see asked over and over again.
In Lightroom, you crop to an aspect ratio, not a size. Size is determined during output (such as a print from the Print module or when creating copies during Export).
Every photo has an aspect ratio, which is just a way to express the relationship between the long side and the short side. A square photo has an aspect ratio of 1:1. As soon as you've have one side longer than the other you have a rectangle and the aspect ratio changes accordingly.
A common photo aspect ratio is 2:3. You can determine an aspect ratio by dividing the long side by the short side. So, for a shot from a D200 is 3872 x 2592, so if we do some math: 3872/2592 = 1.49, which we can round up to 1.5. So, our aspect ratio is 1:1.5, which is just another way to say 2:3.
So for example, if you want to produce a print that is 5 x 7 you would first crop your photo to match a 5 x 7 aspect ratio. Press R to jump to the Crop Tool, then in the Toolbar click the Aspect Ratio drop down menu and choose the 5 x 7 preset (or for a different aspect ratio you could choose Enter Custom and enter the dimensions that match the aspect ratio you want to use). Lightroom will set the crop rectangle to match that aspect ratio. Click the Lock icon to close it, which locks in that aspect ratio so that if you move the resize handles you won't change the aspect ratio. Once you've finalized the crop you can switch to the Print module and create a layout with 5 x 7 cells and the photo will fit snugly inside the cell.
If you want to send the photo away to be printed you would select your photo and go to File > Export. On the Export dialog, choose your file type in the File Settings panel, then expand the Image Settings panel and check the Resize to Fit box. Click the Resize drop-down menu and choose Dimensions, then enter 5 x 7 in the fields below it and set the units to Inches. Enter the desired Resolution value and configure other Export settings as desired. Click Export to have Lightroom create a copy of the source file that matches the settings you've chosen.