Render target size larger than backbuffer?

I noticed that when I use a render target larger than the back buffer, it causes the texture to garble and sometimes causes a crash. I'm using: zbuffer of 4096x4096 A back buffer to match the original screen resolution(1024x768) It works fine unless my render target is larger than 768x768 for example - exceeding the bottom of the screen.. Is that normal? Is it possible to render to a target that's bigger? (ie: 2048x2048)