You cannot fubar something by going too large, other than maybe a $2 spark plug. Going too lean you risk fubaring your whole engine.
The main jet is a screw with a hole in it, which fuel passes through. A larger hole means more gas passing through it (pressures being equal before and after jet change). The carburetor works by sucking in air, which by its motion sucks gas up through the main jet, atomizes it, and the whole mixture flows into the combustion chamber. A larger jet will mean more fuel in the fuel/air ratio, also called running "richer". Smaller jets mean less fuel in the ratio, meaning "leaner" conditions.
If you run too rich there will be more oil and gas in the cylinder than there is air to react with, meaning some will be unburnt and will get on the spark plug and foul it. If your plug is black and wet, its a safe bet youre too rich.
On the other hand, two-stroke engines rely on the relatively cool fuel/oil charge to cool off the piston and cylinder, and if the engine doesnt get enough then it gets too hot and too poorly lubricated, and your engine seizes, destroying your top end, maybe your bottom end, and possibly resulting in a crash. When in doubt, always run rich.