If there's no exposed scaling parameters on the projection node you should be able to use a transform node to scale the input image to the right proportions within a square - youll have to give it the original resolution and do some mild sums to work out the correct scaling but it won't be too tricky.
Expose an int2 at the graph level, put the pixel dimensions into that, use it to work out the aspect ratio of the original image and use that to transform the input before you send it to the projection node $size returns the size of the graph/input node in pixels, not the pixel size of the file you input - designer doesnt…