I have mine set to 1440 x 900 in Steam and the worst that happens is the corners are chopped off - but that's due to draw range limitations. I can't confirm nor deny whether using a non-standard aspect ratio can mess with the way the client renders though.

You need to reduce your "forced game size" in Razor. I think the maximum acceptable level before the client starts acting weird is around 1024x728, maybe a bit bigger than that, haven't tried.

You can set a hotkey in Razor to resynchronize the client, which gets rid of current black boxes. New ones will still pop up if your game size is big enough.

As others have said, try reducing the size little by little until it's tolerable.

What are these black boxes and how can I fix them