GAMA venting tables
http://0323c7c.netsolhost.com/docs/gama%20venting.pdf
I'm not certified or licensed in handling gas appliances, but I figure the Gas Appliance Manufacturing Association and American Gas Association should have it right. ..I noticed the revision is from 1991 but I figure you would at least know if you are way oversized with the vent and would therefore possibly require a duct sleeve to be added.
Also be sure you have double-wall B-vent pipe and not a single wall pipe.
Hope this helps.