in general, the finer the grind the faster and more thoroughly it will vape, because there is more surface area exposed this way. Its like pan frying loose ground beef compared to frying the same meat in thick burger patties - the loose meat will cook quicker and more evenly throughout, while the thick patties may get scorched on the outside while still being raw in the middle. At least thats how i'd describe it, but maybe i'm just hungry.
as AGbeer said above, it really depends on your vape as well. For something like the MFLB, or any vape that uses conduction at least partially, the finer the better. But for most desktop vapes its not as essential to grind quite so fine. Also, most desktop vapes dont have that fine of a screen like the MFLB so theres a chance stuff could get sucked through if ground too fine.
For use in my SSV, i just grind things up until they fall through the holes in my vapeworld 4 piece grinder. Its about the same grind as what i've always used in the past to roll J's with. The bits are about as big as, say, an uncooked grain of rice.