I have Maple and Cherry Mojobricks
And I cut them into 6 pieces. When I spoke to Fred Grosse last, I believe he said that is the size of the Bar-B-Qubes.
I use them just as you would use chunks in my Backwoods, Big Green Eggs, WSM's and Drums and have been very satisfied with the results. So 3 Qubes should be enough for your Butt cook.
Bigmista uses them!