mp4 is a compression format based on specific blocks of pixels and the legal sizes are set by the codec, there is nothing you can do except pick another codec if you must have 1500 X 1000 as a final size. Why did you pick that size? If you must deliver an mp4 then you should be working with standard sizes for your compositions and they should be square pixels... Always square pixels for mp4.
H.264/ MP4 uses "MPEG Levels" to determine specific optimized compression algorithms at predetermined resolutions. Your comp gets squished because you are using an unsuitable preset with a MPEG level that is constrained. You need to use another one that allows to set the resolution freely.
Ah, I got it. I choosed H.264 as the format in the modul-options