The barbs he has listed are 3/16" (says so on the parts list). However, he's going about it the long way around. You can get a valve already made up for this instead of trying to find a ball, spring, and bolt, to fit in a tee.
Want an even simpler solution? PM me and we can arrange to meet (I'm in Brownsburg). I have several of these already assembled. I'll get to see your car and you'll get a free boost controller. What a deal!