In my opinion, the best Mr. Collins is in the 1980 BBC version of P&P. He is tall, and physically what would probably be described as heavy looking (there is debate as to that just refers to manner or to appearance also, and I think most people who read it take it a reference to looks--after all, it does say heavy *looking*), and captures that great seriousness and formality of manner that the original has. He probably looks older than 24, but it would be difficult to find a good actor who was that young and was able to carry the part off, myself.
Here's a you-tube video of the Collins proposal scene.
Here's a you-tube video of the Collins proposal scene.