If you are trying to wring out every last FPS out of it or maybe serious long distance target then I say neck size
but there is a lot more to brass prep than that.

Some of the guys shooting the 208 amax out of a 308 win do whatever they can to get that bullet going as fast as possible.
Long barrel, throated to hang the bullet out for a long COL, fire form and neck size which does make some difference with the right brass like win brass that has a large case capacity to begin with.

Some guys are getting around 2750 maxed out from the above setups over RL17.

I full length size my hunting ammo and with the 208 amax and with new brass I am getting 2550-2600 fps with the 208 gr amax out of a 22" barrel and that is fast enough for me.