You have to cut it pretty small at that age, which is why I don't bother and use NV for a few weeks, until they're old enough to take bigger chunks.
I can't tell you an exact size, I just eyeball it. I look at their little heads, little teeth, and little throats and do the best I can.
If you're using a meat cleaver, and if you have a lot of pups, I can tell you that you will get blisters trying to cut through that much bone every day.
At least a peddler like me does
Jack