chimney cricket

Not much to measure. It's as wide as the back of the chimney and the sides are steep enough to shed water and leaves. A 6/12 pitch is probably at the lower end for leaves, but more than enough for the water. Or you could match the pitch of the existing roof it will be sitting on.

Unfortunately it's not a trivial exercise once the roof is in place. Cutting into an existing roof and getting the valley flashing right requires you to open up a good sized working area. Cutting into the masonry and installing the chimney counterflashing, and getting it right, is not rocket science, but you must get all of the details right.