For when you don't have imagemagick on your development machine for whatever reason. This goes in your rails model and only applies styles if your in the Production environment. Also uses some stored S3 credentials for AWS S3 storage.

paperclip_conditional_resize.rb

class ModelName < ActiveRecord::Base
  has_attached_file :avatar, :styles => lambda { |attachment| ( Rails.env.production? ) ? { :medium => "104x104", :small => "60x60" }  : {} }, :storage => :s3, :s3_credentials => "#{Rails.root}/config/s3.yml", :path => "/:style/:id/:filename"
end