Also mesh size (wire count) isn't the only factor. Wire diameter is a consideration.
50 x 50 x .009 will make quick work of dried trim in a jar. The first batches will be the best and the longer you aggitate the more 'junk' you will get. But a subsequent screening of the later siftings will remove the 'junk' and leave you with the 'blonde'.
Going for nothing but the 'smallest' dust isn't a productive activity, IMO. There is a lot more 'stuff' to be had, that won't 'drop' through a 100 x 100 x .012 screen.
And the first shakes don't yield the trichrome stalks which are very potent in comparison to other plant parts. These can be gathered in the later shakings and are very worthwhile.
IMO there is no reason to try to collect kif from well trimmed buds. It's something to do with trim. Grinders with 'kif catchers' are a bit of a joke IMO. Remember I live where people openly grow in their back yard and buds are buds and trim is sometimes ( I know a guy) just thrown on the compost pile.
OBTW... "i know a finer screen can produce a more potent kief, but what size is that exactly...?" I really don't see why you would say that. Trichromes obviously develop (grow larger) as the trichrome ages. So the smallest ones are the youngest, or in an area that has the least light and nutrient uptake. Since trichromes mature (ask any grower with a microscope who uses it to judge trichrome color as a sign to harvest)) and since they enlarge with time, the most potent trichromes are not going to be the 'smallest'. And since 'quality' seems to be a factor of color, no screen sizing will yield 'better' kif, IMO. Get a screen that will do the job in a reasonable time frame and resift to remove 'junk'.