Pipe length depends on what revs you want to run the engine at and what you're looking for. All a pipe does is ram more fresh mixture into the cylinder by first drawing some of it out of the cylinder through the transfer ports into the header pipe and then ramming it back through the exhaust port using a reflected positive pressure wave just before the exhaust port gets closed by the piston. This "supercharges" the engine and gives more torque which leads to increased HP. If you run the engine at the revs where it gives peak HP then this "supercharging" will give more HP allowing you to swing a bigger prop at the same revs (maybe higher pitch to get more airspeed from the model). This needs a pipe that's about as short as it can go, tuned for those peak revs.

But you can also use a pipe tuned for much lower revs to give a big boost in torque to swing a much bigger prop like they do in RC aerobatics where speed isn't wanted but great pulling power is needed. The pipe is then much longer so it's tuned for those lower revs. The HP it develops at those lower revs won't be as much as one tuned for peak revs but the torque developed will be far higher than an unpiped engine.

Another thing a long pipe does (when tuned for max torque) is it acts as a speed regulator. If the engine unloads in a dive the engine tries to speed up which puts the pipe out of tune and this drops the available torque quite dramatically (switches off the "supercharging" effect) so then there's not enough torque to turn the prop any faster. This effect is also used in CL stunt to keep the airspeed of the model constant through manoeuvres, giving heaps of power where needed (climbing vertically) but cutting back on power when diving vertically.