Flat slide Carb KEIHIN Fits Stock Intake Boot

What size? .

Not sure I understand what to measure. So you want to know what the ID is of the sided that goes in to intake boot. Or is it the OD Sorry dont know much about the flat side I usually only mess with stock for the blaster. Im selling this for a another guy

Just fig since it fist the stock boot one of my blaster buddy would wont it before I list it on ebay
 
if it fits the stock boot, and it clearly says "pwk" on it,
it's most likely a pwk 28mm