Note that this validation runs both after the file is uploaded and after CarrierWave has processed the image. If your base uploader includes a filter to resize the image then the validation will be run against the resized image, not the original one that was uploaded. If this causes a problem for you, then you should avoid using a resizing filter on the base uploader and put any specific size requirements in a version instead.
So instead of this:
require 'carrierwave/processing/mini_magick'