As ManiacJoe says, to make photos an exact size in pixels, you'll crop to your desired proportions in LR and then specify size in pixels in Export. I'd suggest using Quick Develop in the Library module to get to the same proportions. In Grid view, select all your photos, then in the Quick Develop panel click on the triangle to the right of Saved Preset and when it appears, click on the dropdown next to Crop Ratio, Enter Custom, and type in your proportions. (If you want, for example, 2582 pixels wide by 4527 pixels high, then type in 2582x4527. LR will change this to 2.582x4.527, but that's OK because these are the same proportions. Once you hit OK, LR will crop all your photos. Next, if necessary (maybe not), go into the crop tool in Develop (R) and go photo by photo fine tuning which portion of each photo is cropped off. Finally, export all of them, setting pixel size in the Resize to Fit section. Here's a video tutorial that shows using the Quick Develop panel for quick cropping of many photos.
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