How to resize image on upload in rails app using paperclip?

How to resize image on upload in rails app using paperclip?

In my Rails app, I have the below:
has_attached_file :image
def picture_from_url(url)
self.image = URI.parse(url)
end
I would like to resize the image when it gets saved as well but am not
sure how to do this. I would like the width to be 300px and the height to
scale proportionally to this. Any advice on how to do this?