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Use user option string field.
xres 100 yres 150
Notice the 2 are on same line, with a space in between, you can stretch the pixels on x and y like this.
Let me know if this is cutting it for what you want.
Thank you Mads for your answer. Seems to be that. I see also there is pixel_aspect_ratio available through user option string field. I can do some tests now I know where to look at.