title: validate size of upload using Carrierwave
body: <p>In our latest application we need to process some uploads, I've worked with paperclip before and everything just works! but we're giving <a href="https://github.com/jnicklas/carrierwave">carrierwave</a> a try, it looks promising but, I can't find <strong>how to validate the size of an attachment</strong>, it seems like the <a href="http://carrierwave.rubyforge.org/rdoc/">documentation</a> doesn't have any information about it, should we add it manually to the model via a custom validator?</p>
  
  <p>Thanks in advance!</p>