there is a number stamped on the side of the jet...example 68.
I change jets 2 sizes at a time until it's where I want it. You can go by the plug reading, mph(race car) or use an EGT to decide which way to go. Street car I'd just jet it to where it runs the best without loading up at slow speeds. Make sure the power valve is correct for the amount of vaccume you have at idle in gear.