Firstly, I agree re: screen-edge scrolling. It's the first thing I tried to do.
Secondly, I like it. Clear, simple gameplay. I like RTS games and also browser games that don't use plugins. Also lots of potential to expand, add maps, features, etc, down the road.
Thirdly, I suggest pre-loading all your images. For the first little while of playing, half the vehicles show the "image not loaded" symbol when they turn to a certain angle whose picture hasn't loaded yet; Or else they show the gradually-loading-downwards effect of web-images.
Fourth, I would reduce the scale just a bit, so that the tanks/cars are maybe 50-70 percent their current size, so you can fit more on screen without scrolling.
Lastly, I know how to maximise my browser, and don't want to be told to do so. You could say on the loading screen "Works best if your browser is maximised" or something, but leave it up to the user.
Keep up the good work.