You may define the size of exported pictures in pixels.
This can be set in Preferences: General tab for all exported images.
For any individual preset, you may override the general defaults by selecting special parameters of the presets themselves.
This can be done when you create a new preset with the Crop Setup Panel. There are Fixed Pixel Export Width and Upscale smaller images options.
Fixed Pixel Export Width - If this field is zero, the program exports the crop as large (or as small) as it was cut from the photo. If this field is not zero, the program downscales larger crops to the selected size.
Upscale smaller images - If Fixed Pixel Export Width is not zero and this option is active, the program upscales smaller crops and downscales larger crops to the selected size.
Alternatively, you may select these parameters by editing presets as a text file. Here are the lines that you should edit and an example of usage.
#FixedOutputWidth:<NNN> // Fixed output width (Integer value)
FixedOutputWidth:0 // Export with original size
FixedOutputWidth:1000 // Downscale bigger images to 1000 pixels
// Smaller images in original size
FixedOutputWidth:-1000 // Downscale bigger images to 1000 pixels
// Upscale smaller ones to 1000 pixels