Step 2: Get yourself a turbo charger and hide away in the garage building your insane jet powered contraption!
As a general rule, it is not so much the size of the whole turbo as it is the size of the inducer that matters. The inducer is the visible area of the compressor blades that can be seen when looking at the turbo's compressor with the covers (housings) on. Looking at the turbo here will show that the air inlet is quite large at almost 5 inches in diamter, while the visible blades of the inducer are only 3 inches in diameter. This is plenty for creating enough thrust to drive a mini motorcycle, go kart, or other small vehicle.
The turbo in the picture is a Cummins ST-50 off of a big 18 wheeler truck.