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Backpressure is bad unless you like using engine power to forcefully expel exhaust out of the engine against its will.
Not sure what you mean by Pulse Width.
Diameter of pipe affects the backpressure of the system but also the speed of exhaust flow through the pipe. Larger diameter decreases backpressure by reducing resistance to gas flow but the flow slows down.
Length of the pipe in any area, combined with the diameter, determines how you tune the acoustics of the system so that a low pressure wave intersects the exhaust port during valve overlap to scavenge the exhaust from the cylinder and prevent reversion flow. Length cannot be set in a vacuum as the speed of flow as defined by diameter and engine size (mass flow rate) informs the length choice.
Length/diameter have to be paired. Longer primary tubes tune for better exhaust scavenging at slower engine speeds. Larger primary tube diameter makes exhaust flow more slowly and will tune a given length for an even lower engine speed. Excessively large pipe diameter makes the flow slow down too much, lose too much heat (changing the speed of sound in the gas), and puts you well past the point of diminishing returns on increasing pipe diameter. Length and diameter are obviously constrained by packaging concerns in most cases so compromises are made such as tuning for a harmonic of the desired tune frequency (at each harmonic of the tuned speed you see varying percent of the full benefit of the actual tuned frequency).
Muffler design is another ball of wax from tuning for maximum flow (or acoustical supercharging). It's a more standard noise reduction technique and each method of reducing noise will do different things to the upstream pulse tuning depending on the physical design. Mufflers are typically resonant chambers often broken down into smaller or multiple differently-sized resonant chambers inside, often with Helmholtz or quarter-wave resonators inside and many take advantage of insertion losses associated with passing the flow pipe some distance past the flat plate entry into the chamber. Other mufflers use perforated pipe and deadening material within the chamber to dampen pulses (glass packs) and still others use pulse reflection and resonant volume to reduce overall sound transmission. Too many options...much more than any OEM can handle to optimize for any one enthusiast's desired outcome under the restrictions of cost and time.
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