From what I've read on these forums, a lot of people skip the XS sandies. For budget, many people begin with prefolds. Others start in with size Small Sandies or the OS diaps. Personally I started using cloth when my DD was two months old, and she was already into the size small sandies (she was 10 lb 7 oz at birth).  I'm really glad I used sandies, rather than prefolds, because the few times I experimented with prefolds the BM spilling onto the diaper covers (thankfully the BM never leaked out of the cover onto the clothing, though). 

That said, I think the sizes being sold second-hand tends to vary month to month. Earlier this year I remember seeing more of the XS and S sizes, but now I've noticed more of the larger. Good luck!