Please help! What jet?

I am getting a pro circuit t-4 exhaust for my 250ex.
I believe I need to rejet.
Ca you guys tell me what size jet to go to?
Or where to find info about this?
Thanks

Or do I even need to rejet?

[TRX_Thumper]

I'm not sure what your stock size is but stick a much larger in it and if its rich work your way down till the plug is brown.