The ‘official’ (or nominal, if you like) capacity of the balloon is 300 cc.
You can easily calibrate your inflator pump as follows /this is how I did it/:
– Take a measuring jar, preferably a larger one, say 1 litre/quart
– Fill it to 400 or 500 cc with water
– Connect the inflator bulb to the device with valve open
– Squeeze the balloon completely with one hand and hold it while you close the valve on the inflator
– Immerse the flat balloon to the water compeletely
– Open the valve and start to inflate, counting the pump strokes
– Monitor the volume level in the jar
Obviously you can perform this without getting the balloon flat; I insert it flat and so had to get the count from that state.
For me it took 10 or 11 strokes to get to 300 cc; to be honest I feel it a bit too big in my rectum, so I usually do 8 only.