When I change the size of thumbnail, it is hard to keep ratio of thumbnail.
I usually use 16:9 or 1.85:1 ratio images. (XnView's preset for thumbnail size support only 4:3 and 1:1 now)
So When user change size of thumbnail in Option>Browser>Thumbnails Menu, I think ratio option is needed.
Set Ratio first and change width of thumbnail, then automatically set height.
that is my Suggestion.
Thank you.