Schedule 40 6 inch what lenths can I purchase?

Comes in 10' or shorter, mainly due to shipping issues. If you need more its very easy to weld on another section. Most suppliers will sell 3', 4' 5' and 10' only

In any areas I have worked, 6" PVC, schedule 40 DWV pipe is only sold in 10 and 20 foot lengths, so that should satisfy your 16 foot requirement easily, Cjbond. You will only find this at a plumbing supply wholesaler... ;)

Mark