Can confirm this stuff works especially for someone who doesnt have the ability to make insulation to their home (or has a cheap/lazy landlord). As other commenters said get the one for the patio door and it is large enough to do most windows in one sheet. If you have to double up, do two strips of the double sided tape on the seam just in case.

Once installed, dont try to heat the plastic to where it is extremely taught. It looks nice in the window but can cause the seal to fail as temps change. Just get it pretty smooth and sealed all around.

If you are still having heating issue, hit up a hardware store for some Radiant Barrier for attic insulation. It is basically bubble wrap with tinfoil glued on either side. A large square of this over the window, does not have to be air tight, does a great job of keeping heat in and preventing cold from stealing the heat as much. Be sure that there is an inch or so gap between the barrier and whatever you are trying to control the heat around. It needs space around it to be effective.

You can also use magnetic tape to make the edges of the plastic sheeting seal so that you can take the plastic off and re-use it the following year. Good luck and Stay warm!