Native Shopping Ads are designed as a responsive and dynamic ad unit that take up optimal space based on the number of product recommendations and the amount of space available in the container where it is placed. The recommended placement is at the end of a product article or within a product article page and NOT the side rail. For these placements, the customization option that we would like to provide to you is the number of rows as that would make it very simple to configure without you having to worry about the number of pixels. It would also ensure that the ad unit continues to be responsive and works equally well on all mobile devices.

But I would like to place it in the side rail and hence I would need to customize the width and the height of the ad unit?
Native Shopping Ads are NOT designed for side rail placements. We do not endorse or recommend such placements as we have found them to be sub-optimal for this ad unit. It is only the Custom Ads variant where we provide you with some additional flexibility to specify the width and height in case you want to place it in a specific way. If you need standard IAB sized ad units then we would suggest that you choose from the wide variety of banners available from Amazon Associates.
Note: Native Shopping Ads are responsive unit. So, they would work even in the side rail but they are not designed for side rail.