The diameter of the tube and the wall thickness depends on how far you wish the tube to be un-supported as well as how big your cruising chute is.
I would think you would want to push the bow sprit out between 1 and 2' clear of the bow so you can get away with seemingly light material.
Don't forget to put a pad on the foredeck though - otherwise the inner end could end up denting your boat ... /forums/images/graemlins/grin.gif