My AR has a 1 in 8 twist and was hoping to get a few ideas what bullet weight I should try first. (factory ammo for now)
Th beauty of a 1-8" twist is that it will shoot most any weight bullet pretty well (with the exception of some very heavy long range target bullets).
I'm assuming you are talking about a 5.56/223 Rem. gun.
I've had 1-8" barrels that would shoot 40gr varmit bullets into sub .5 MOA groups, and still shoot 69gr into little bitty groups as well.
Each barrel will have it's personal preferences on type and weight of bullets - you will just need to try a few different ones to see what your gun likes.
Bullet length has more to do with it than weight, depending more on the length of the bullets bearing surface that is in contact with the rifling/grooves.
What a bullet is made/constructed of will change the length.
As an example a solid copper constructed bullet like a Barnes will be longer than a bullet with a lead core of the same weight.
It can't get any bigger around because of the barrel diameter, so has to be longer to get to the same weight as the heavier lead material.
I'd just get some different weight bullets that you think are closest to being the proper type for your intended application and do some shooting.
Your gun will tell you what it likes for best accuracy.
I'm sure some others will jump in with advice, but it would help if they knew what you want to use your gun for (varmint shooting, target shooting, etc.).