ruby-on-rails - Omniauth Facebook Ruby on Rails image_size

Question

このようにFacebookからデータを取得しています。特定の幅と高さで画像を取得したいのですが、どうすればよいですか?

class User < ActiveRecord::Base
  attr_accessible :password, :username, :oauth_token, :provider, :uid, :oauth_expires_at, :picture, :email, :name

def self.from_omniauth(auth)
  where(auth.slice(:provider, :uid)).first_or_initialize.tap do |user|
    user.provider = auth.provider
    user.uid = auth.uid
    user.name = auth.info.name
    user.username = auth.extra.username
    user.email = auth.info.email
    user.picture = auth.info.image
    user.oauth_token = auth.credentials.token
    user.oauth_expires_at = Time.at(auth.credentials.expires_at)
    user.save!
end
end
end