Rails Gravatar Helper Method

I have gone through this railscast on Gravatars and i now have the below helper method in my application helper.

module ApplicationHelper
  def avatar_url(user)
    gravatar_id = Digest::MD5.hexdigest(user.email.downcase)
    "http://gravatar.com/avatar/#{gravatar_id}.png?s=200"
  end
end

and i have this in my view

<%= image_tag avatar_url(user) %>

how can i modify the helper so it will accept a size option that changes the s=200 to the size specified?

Thanks

module ApplicationHelper
  def avatar_url(user, size)
    gravatar_id = Digest::MD5.hexdigest(user.email.downcase)
    "http://gravatar.com/avatar/#{gravatar_id}.png?s=#{size}"
  end
end

Then call:

<%= image_tag avatar_url(user, 200) %>

You can also check Michael Hartl's guide.