It depends. You may not need one at all. Put it all together then measure.
Also do a search uner my name and you find the same question.
Mine had a shackle reversal done so that changes things. Bottom line is you do not know until you put it together. It may be fine.
Back when I ran leafs, I first used the 2" alum. shims on the front. They eventually failed & worked their way loose, causing my bottom spring mount hole to "egg". Didn't use any on the rear. I'd suggest steel shims, but it's been so long ago I forgot where I got 'em.