I have a custom column of #pages and the values vary between about 100 and 2500. Most of them are between 200 - 500. None of them have negative values,

. Others' values should differ.
So I think you will find it hard to entertain users by giving a fixed set of values. I personally only care for: Short, Medium, Long or Very Short, Short, Medium, Long, Very Long. If you can provide something like that and let user define by themselves the ranges of words or pages under each category, it would be nice.
And if I could achieve this in Calibre, I wouldn't need this feature added because it would show up anyway.
Edit: in Calibre I could achieve this by using "Saved Searches"