Size and weight of a disc both make a difference to the amount of energy it stores, as does speed. Here is a useful calculator: http://www.botlanta.org/converters/d.../flywheel.html

Spinning discs are VERY dangerous. I would suggest not building a spinner unless:

1. You have a safe place to test it. [I tested my first one in an empty barn while sheltering behind a 12 inch brick wall.]
2. You are confident of the strength of your mount. If a disc breaks free of its mount it can go a long way and even a modest one can be lethal.