# In your controller parse a "size" parameter
class UsersController < ApplicationController
  def show
    @user = User.find(params[:id])
    @size = params[:size] == 'original' ? 'original' : 'small'

    respond_to do |format|
      format.html # show.html.erb
      format.xml  { render :xml => @user }
    end
  end
end

# In your view, display the appropriate size, and a link somewhere to the other size.
# You need "to_sym" since Paperclip expects a symbol... :original, not 'original'
<p>
  <b>Avatar:</b>
  <%= image_tag @user.avatar.url(@size.to_sym) %><br>
</p>
<%= link_to_other_size(@size) %>

# Hide your logic for getting the "other size" link in a helper method:
module UsersHelper

  def link_to_other_size(size)
    case size
    when 'small'
      other_size = 'original'
    else
      other_size = 'small'
    end
    link_to link_name_for_size(other_size), :size => other_size
  end

  def link_name_for_size(size)
    "#{size.capitalize} Size"
  end

end