I mostly ride in urban settings. My experience was that the fender length didn't make much difference in the rain because I got splashed by cars, buses, and trucks anyway. For city riders, IMHO, the main purpose of fenders is to shield the bike from road grimes and water puddles. In such cases the determinant of ideal fender length would most likely to be how "dirty" your regular routes are. For instance, when I lived in NY I had full-length fenders on:



After moving to SF, I found that short fenders were good enough for my needs:



(BTW, these delightful wooden fenders are handcrafted by Mr. Pual Sykes. Thank you Paul.)