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Hi there!

This is not possible at the moment, due to the render canvas size being hardcoded in order to reduce the number of things that can go wrong with it. It might be looked into the future but with a big red warning that using canvas that aren't powers of 2 can cause from little artifacts to random crashes. 

EDIT: I'm sorry, I thought that you meant to change the canvas size to 450x450 (I shouldn't read that fast). You might be using the HD mode, it increases the output resolution in order to achieve the HD look. In order to get a 512x512 canvas you should use the pixel art mode.