file_uploader.rb

 #carrierwave, get image width and height of image (using the MiniMagick gem)
 
 #stick this in your uploader (ex: uploaders/file_uploader.rb)
 def width
    @image ||= get_minimagick_image
    @image[:width]
  end

  def height
    @image ||= get_minimagick_image
    @image[:height]
  end

  def get_minimagick_image
    MiniMagick::Image.open(Rails.root.join("public/#{self.url}"))
  end

 #now you can call it like this from anywhere
 #file is my uploader object
 @image = Image.find(1)
 @image.file.height