I didn't chop them up, personally, but I used mini-buds, which were kinda smallish anyway. If I used real (large) buds, I'd probably cut them up into inch-sized pieces at least. But I doubt it really matters much if you stir them up reasonably frequently. And don't be too gentle, since in this case you WANT to knock all the trichomes and stuff loose into the oil. Stir vigorously.