Paperclip image dimension validator from http://stackoverflow.com/questions/12388887/paperclip-image-dimensions-custom-validator

dimensions_validator.rb

class DimensionsValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    # I'm not sure about this:
    dimensions = Paperclip::Geometry.from_file(value.queued_for_write[:original].path)
    # But this is what you need to know:
    width = options[:width]
    height = options[:height] 

    record.errors[attribute] << "Width must be #{width}px" unless dimensions.width == width
    record.errors[attribute] << "Height must be #{height}px" unless dimensions.height == height
    end
  end
end

validates :file, :dimensions => { :width => 300, :height => 300 }