Since you mentioned using it from inside your car, you might consider removing the 9 volt battery and using your car's battery (wired through a 12 volt to 9 volt reducer - of course). Wire it fused-direct or through the lighter socket. That will probably reduce the size by half. Also, you might get more range by improving the antenna at the receiving end.
We stretch the laws of physics when we build state-of-the-art but there are limits when you want both smaller and more powerful in the same package.