"I would like a back up string so how do i order one? Do i use the measurement of the bow or the measurement of my current string?"
Depends on who you are ordering from. Personally, I prefer the actual string length. AMO says 3" shorter for longbow or recurve, but a lot of bows don't follow AMO specs.
"I bought a used bow online and it did come with two...I have been shooting the one outfitted with silencers but the other is actually newer but is longer??? I may have just got a too long string."
If it's a lot too long, you got the wrong string. If it's a little too long, you can twist it up.
"What type string do you recommend? I can shoot fast flight, is that worth it?"
I like Dynaflight '97 or 8125--both are "fast flight" type materials. I think they are well worth it. The speed difference doesn't matter to me--I'd use them if they were slower. The difference that matters to me is the lack of stretch/creep and increased durability.
Keep in mind that you need your back-up string to be the same material, strand count, etc. as the primary string. There can be enough difference in the two to need to re-tune the bow, or even change arrows or at least point weight.
Chad