Specify image sizing

 If you choose JPEG, PSD, or TIFF as your file format in the Export dialog box, specify the image size.
Resize To Fit
Sets a maximum size for the width or height of your photos, which determines how many pixels the photos have and specifies the fineness of detail in the exported images. Select an option and then specify edge measurements in inches, centimeters, or pixels. If you check Don’t Enlarge, Lightroom respects the resolution you specify and disregards width or height settings that would enlarge the photo.
Width & Height
Resizes the photo to fit within the specified width and height, retaining the original aspect ratio. Specifying 400 x 600, for example, produces a 400 x 600 portrait photo or a 400 x 267 landscape photo.

Dimensions
Applies the higher value to the longer edge of the photo and the lower value to the shorter edge regardless of the photo’s original aspect ratio. Specifying 400 x 600, for example, produces a 400 x 600 portrait photo or a 600 x 400 landscape photo.

Long Edge and Short Edge
Applies the value to the long or short edge of the photo and uses the original aspect ratio of the photo to calculate the other edge.

Note: The long edge of an exported photo cannot exceed 65,000 pixels.

Resolution
Sets metadata within the exported photo that describes how large the photos appear at a given resolution.