I tested my dns servers with the oarc test and my size limit is at least 1403 bytes. I performed the same test before my Juniper ISG 2000 and the result is 2047 bytes.
This article talks about
ip virtual-reassembly for cisco but I can't find the equivalent for Juniper.
I prefer to find the good option in JunOS before I talk about this with my net admin :-)