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I'm certainly no expert, just few youtube vids here and there ..
Changing diameter changes exhaust gas speed, bigger - slower, smaller - faster (to later if too small also extra resistance to flow?).
Length together with diameter, both affect how fast exhaust gases reach runner merge, from that one can get "free gains" by enhancing exhaust scavenging .. there was something like when pulse from one cyl reaches merge, it should help scavenging from "paired" cylinder, enhancing scavenging & fill efficiencies.
Well tuned for use type exhaust & intake manifold tube lengths & diameters is common way to get extra performance, especially on NA engines. Matters a bit less for forced induction, where one can just up boost.
Unfortunately, just as with intake tube diameters/lengths, as gas speed is linked also to engine rpms, intake & exhaust tube length & diameter usually can be tuned just for some specific rpm range, which is usually selected by intended engine/car use type, eg. high-rpm race, low-rpm daily driving, or compromise inbetween. Good examples - PTuning short runner header with high absolute gains in top but relatively little in mid & low. Longtube headers for twins seem doing well in "mid"/overall, also UEL headers, that compromise length "tuning" for cylinder pairs in fashion of "jack of all trades, master of none" .. but also easing clearance, lessening material costs (and for some also providing wishing specific sound change from "subaru burble").
Most advanced (but also most expensive and difficult to design/make) intake and exhaust bits are of variable length (via valve-switching between two sets of different tubing length for example, or changing length of tubing itself). Due cost/complicate variable length intakes/exhausts design rarely used (maybe in some racecars? supercars? motorcycles?).
When best tube length & diameter gets known, be it by some calculations/prior statistical data or simple experiments by selecting different length tubes to mock up test headers, there unfortunately comes in packaging problem, more pronounced in today's cramped engine bays. This may strike out some most effective configurations or add some other issues (eg. how with long tube headers Ace may get some overpipe clearance/rubbing issues, or Nameless some heat radiation/melting some parts issues).
So at the end it's often about choosing best compromise for use type within budget and design/manufacturing/spacing limitations.
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