Widget depending on page size

Hello,

I’m currently running a blog for my girlfriend and I have advertisement on it, What I want to do is make the ammount of ads dependant on the page length.
So when A single post is loaded that doesn’t have much content on it no ads or slighter ads are loaded than when a verry long post is opened, in which case it fills the column space up as much as possible.

Is there a plugin or conditional tag(what I’ve been using so far) for this?

Thanks in advance!