There's really no such thing.
A jet kit includes a variety of different main fuel jets, that should fit your intended application, given what the jet kit provider knows about the make, model and average mods of your bike.
The proper main jet is the one that maintains optimal fuel mixture throughout the rev range.
Since an internal combustion engine is basically an air-pump, most mods are intended to increase the amount of air the motor can move per cycle, and thus increase horsepower (more air + more fuel = more bang).
So, you generally increase the main jet size to compensate for the increase in air, thus maintaining an optimal fuel ratio of approx 13 or 14:1.
There's a lot of better information available here:
With a filter and pipes only, you may go up 1 or 2 main jet sizes. The instructions from the kit maker will provide a recomendation.
But, every machine is different. That's the thing about carbs - once you mess with them, they can take a lot of time and trials to get right again.
Just buy the FCRs, if you can afford them. The performance increase is so far beyond what you think they can do, it's amazing.