This is good! I personally set the Custom thumbnail size to a square ratio. The resulting square container means that landscape-oriented images don't display larger than portrait-oriented images in the browser, giving both of them equal visual weight**. In sum, by doing that, my XNView browser becomes a modern-day equivalent of the light-table for yesteryear's film slides.
I suggest here adding such a function at Create > Contact sheet... > Settings.
Currently, in the Contact sheet dialog, users can adjust how many thumbnails there are per page and how much they are spaced, both of which affect the size of the image, but because there is no "Custom thumbnail" dialog, users can't change the shape of the container, and thus landscape-oriented images are "favored" over portrait-oriented ones, the former appearing larger than the latter. Thus, having a Custom thumbnail function in the Contact sheet settings would allow users to adjust things to their liking there as well.
Thanks for hearing me out!
* Thus, as an aside, I would also suggest rewording this function as "Custom thumbnail-cell size", "custom thumbnail-container size", or "Custom thumbnail-holder size".
** Du coup, c'est les images carrées qui en profitent, mais elles sont moins nombreuses.
