IMO it's all to do with proportions....you want to top of the boot to come to exactly halfway between the floor and your knee. So with flat boots like uggs, it's best to go for the short ones. With a heel, longer boots are more flattering as they need to be that bit taller in the boot bit to hit the midway point on your leg. Same principle applies to rolling jeans up/wearing capris.....roll them so they're exacty midway between the floor and your knee, and bingo, instant supermodel legs
wwoow!! somebody's got a degree in Uggs...!!! 
thanx bunny!