The length of hose that you need depends on how your shop is set-up. There are 2 things to think about as far as the length. The length will have an effect on the amount of air available. Like said, the shorter the better. Another thing to think of is trash in the paint. Many times, people will have a long hose that drags on the ground. They lay the hose down when refilling the gun. When the painting is done, they find trash in the paint, mainly on the horizontal panels, like roof, hood, and trunk lid.
Idealy you want a hose that is long enough to paint one side of the car, with another one for the other side. You should also have a set-up that will keep the hoses from touching the ground or car. Also don't buy a cheap hose. You want the largest that you can get, atleast 3/8" ID to provide plenty of air capacity.