I rented a movie on my iPad but there isn't enough space on it to download the movie so now I'm trying to either watch it on my AppleTV or laptop through iTunes, but I can't seem to find how to do that. Is it not possible?
No, it is not possible.
See this Apple Support Document for more information:
You'll have to expand the section on "Movie rentals on iPad" to see the above quote.
Your only option is to free up enough space to watch it on your iPad.
That is a disgrace. My parents bought an Apple TV on the promise of compatibility between Apple devices, and yet now they find that for some bizarre reason the rules are different for their iPad. They rented a film on iTunes on the iPad whilst they were out of the house, and found that it could only be watched on the iPad, unlike films rented on their MacBook which can be watched on all devices. There was no warning or disclaimer about this, only a bit of small print on the we page linked to. I think that is a con.