The factory rear bolts are something like 12mm x 50-55mm.
BUT depending on which shocks you have and what size metal sleeve is in the lower eyelet, you can sometimes cram a 1/2" or 13mm bolt in there.
I'd suggest going with some nice Grade 5 or Grade 8 bolts, and getting some metal locknuts to go with them. I've found ACE usually has a good selection, if you don't have a good local hardware store or Lowe's doesn't have what you need in their Grade 5/8 or metric bin.