Carburator main jet sizes are measured in 1/100ths of a millimeter. A jet size of 120 actually has an opening of 1.2 millimeters in diameter. The main jets come into play at engine rpms of about 2,500 to 4,500 rpms. A beginning main jet size of 100 makes a good starting point. Use larger or smaller jets to obtain the desired result. A larger jet diameter means more fuel will be used. You do not want to use a size so large that raw fuel will bypass the rings and end up in the crankcase.
For more info, see How to Do Car Tuning on the Cheap