gistfile1.rb

class Message < ActiveRecord::Base
  validate :image_size_validation, :if => Proc.new { |m| m.image.present? }

  mount_uploader :image, MessageImageUploader

  private

  def image_size_validation
    errors[:image] << "should be less than 2MB" if image.size > 2.megabytes
  end


end

Ah that's sugar, thanks!

I took your basic idea and turned it into a Rails 3 style validator. It's basically a clone of the built-in validates_length_of validator, but checks the file size instead. https://gist.github.com/1009861

Cool, thanks!