Actually, fine crystals work well. In SWIM's experience, a little TOO well. The fine powder dissolves very quickly through the nasal membranes, and it hits hard and fast.
The reason larger crystals are preferred is because 100mg of fine crystals appears to be much more product than a line of 100mg of denser product. Fine enough, and the stuff appears almost "fluffy". So people do what they consider a normal line, and complain about how weak it is.
It's hard to guesstimate a "point" when the volume isn't consistent.