Ok so im trying to track down what exactly is causing an nbt packet issue randomly on connection to a server. I thought the easiest way to do this would be to have a mod that's client side and just prints packet information. My question though is, is there anyway of getting what mod is sending the packet and what size it is ?  So that i can compare to the packet causing the issue and then track it down further from that.

It is not recommended for normal use in a mod, but for debugging purposes this might help -- basically you remember the channel from the connection event and then try to substitute your own channel handler. I give a bit more detail here: http://jabelarminecraft.blogspot.com/p/minecraft-forge-modding.html but you'll have to do the rest of the work not sure how tricky it is.