Originally Posted by Yooper_Brew
I'd start with an ounce, and see how oaky it gets first. It's very easy to overoak, and then it takes forever to age out. I used 1 ounce in a 3 gallon batch last year, and only oaked it for 10 days, and it took months to taste like I wasn't drinking an oak tree. I used medium American toast.
+1! I have 2 oaked recipes that I do (vanilla robust porter and a rye pale ale) and have over time drastically cut back on the amount of oak that I use because of way over-oaking with larger amounts. I'm down to 1 oz for a 5 gal batch, and I leave it in for ~2 weeks.
Granted, a imperial stout may stand up to the oakiness better, but with something like this, you're probably better to go low and slow, then add more if you need to later (you're going to want to age this a bit anyway, right?)
FWIW, I also use American oak, but I have been using the light toast and I soak them in bourbon before using them.