what size piece of wood does one need to be able to get a riser out of?

it is all in the lenght, I made mine from a piece 21"

how thick does it need to be 6" maybe a piece 6x6x24?

I used 4" less work i was lazy at the time it finished at 2-7/8"thick

Longbow or recurve riser? A 6x6x24 will be expensive if you can find a piece of wood that large and depending on what kind of wood.

when wife gets back with the camera I will send you some pics

pics sent by e-mail

Depends on what you are making. longbow recurve TD or one piece. 8/4 thickness is plenty (2 in.)
8/4 by 4 by 24 ought to be enough to do about any of them.