Length does effect performance.... so the ladies say...
But basically the rule of thumb is long trumpets for torque, short trumpets to top end horsepower.
Design makes a big difference, we sell the Titan Motorsport Billet Trumpets after they made an extra 3.5bhp on my Zetec.
They are a full radius mouth with a 7 degree taper, this serves to give smooth delivery of air with the slight taper increasing airspeed without causing turbulence.
The work out about £24 each.. which is cheap horsepower really and they look the nuts !