ESAB has an excellent book on GTAW. it's called Tig Welding Handbook, and it has all kinds of charts for stainless steel, aluminum, and carbon steel for various thicknesses and it also says what thickness of tungsten to use...u can probably find this book at a welding store, or u can get one by mail:
ESAB Welding & Cutting Products
PO Box 100545
Florence, SC 29501-0545