I certainly trust that you're able to fix it. A render target can be as big as a barn door if your hardware permits.

First check out the render target examples, especially the pssm script, and compare them with your code. If you can not draw into all of the target area, check if you have correctly set up the viewport size with view.pos x/y and view.size x/y. Next thing to check is if you've correctly created a z buffer for your render target, and if it has the right size.

If you tried anything and can still not fix it, please post here again and I'll look through your code.