Sound (pressure) waves move at the speed of sound and that varies with temperature of the gas it's traveling through. The gas itself moves way slower. Large pipes hurt gas velocity and disrupt harmonics.
Long pipes restrict flow and gas velocity, but with pipes that are way larger than ideal, that is not an issue. In fact it helps considerably. Exhaust harmonics are generally well understood by exhaust manufacturers but there's a lot of incorrect info out there. In theory, you want the wave to travel down the pipe and be reversed at the open end and to reach the head as both valves are open to pull burned gas out of the head and to pull fresh mixture in. Then that wave travels back down the pipe and this time it is reflected as a positive (stuffing) wave which should be timed to arrive just before the exhaust valve closes.
In practice that is just not really an available option, so exhaust designers start to look at timing of second and 4th and 6th harmonics as the waves go back up and down the pipe. For us mere mortals, it's possible to get a copy of articles by David Vizard on exhaust design or refer to the book by AG Bell or even better look at some of the more technical publications for inspiration.
And for the rest of us, think of exhausts as short and large let the motor rev but with no guts - just empty noise. Longer smaller diameter pipes tend to sound better and to add torque which is the ability of an engine to do work i.e. to get our sorry asses into motion. That's a gross oversimplification, but close enough.
Long works better. Short looks sharper but doesn't work or sound as good on the road.
