The numbers you're seeing, ie A3 vs Fox #8 are meaningless in themselves, they are just names. There is no "standard" labeling of plugs, and as you noticed with the OS plugs, the size can change a bit too.

As I said above, for the most part, the length of the plug isn't critical for you. If you run some of the smaller engines, you need a shorter plug to keep it from getting too close to the piston (or maybe hitting it), and 4-strokes often require a longer reach plug to get all the way down to the combustion chamber. But for the vast majority of 2-strokes in the "common" sizes, the differnece in length with the OS plugs doesn't matter. However, it's worth noting that the longer plugs do increase the compression of the engine slightly, and some ABC engines when new can wind up "over compressed" with a longer plug and higher nitro. I've seen this twice on OS .46FX engines on 15+% fuel. A longer plug lead to the engine not running right at full power, but a shorter OS plug ran it fine. (I suspect that if we had a colder plug around in those cases, it would have also fixed the problem).

The "important" thing about plugs is the "heat range" of the plug. You'll see plugs referred to as "hot" "cold" or "medium". Most sport engines run just fine on a medium plug, and most of the plugs you pick up will be pretty much in the medium range.

A "hot" plug causes the fuel/aix mixture to ignite sooner, a colder plug causes it to ignite later. It's a lot like adjusting the timing on a spark ignition engine (like an auto engine).

An engine with too cold of a plug will not idle or transition well, the plug isn't staying hot enough at the lower speeds to keep the engine running.

And engine with too hot of a plug will run in to pre-igintion problems. In the extreme, this is rough, ragged running at the high end that you can't fix with the mixture no matter what you try. Usually it's more noticed with a sizzling or "frying egg" sound when the engine is running.

As I said, most sport engines using "normal" fuel will prefer medium plugs. If you raise the compression, use a tuned pipe, run really high RPMS and high nitro, you'd want a colder plug. (and a stronger, heavy duty plug element to keep it from being sucked though the engine).

On the other hand, 4-strokes and engines set up to swing a big prop at a lower RPM will often benifit from a hotter plug. Most "4-stroke" plugs are just hot to very hot plugs with a slightly longer reach to them. You can run a 4-stroke plug in a 2-stroke, and vice versa, but you'll often get the kind of problems mentioned above.

The best way to pick a plug is to try it and see how it runs in your engine setup, on your fuel. If the engine works like you want it to, you're good to go. In theory, you can use an accurate tach to check the engine RPMS, and the plug with the best rpm is the best plug for the engine, as it's causing igntion at the right time. But you're talking about small enough changes that most cheap tachs won't show it anyway. Beyond that, you can buy plugs by price and local availability. A more expensive plug may last longer in your engine or perform better, or it may not. Guys tend to stick with a plug they like just because familiar is good, and once your engine is set up and working right, the less you mess with it, the better.