from your collar to how low on your body you want the bush shirt to end, some people are close to down to the knee (or it looks like that in their pictures), mine is mid jacket length and ends just covering my pockets if i was wearing jeans.
As soon as i put mine on i feel ten times more bushy, have to remind myself that i am still just an enthusiastic novice and this is the bush shirt talking making me feel like i am wearing bush related armour