IR receiver output pulsating instead of constant one('5V') or zero ('0V')
If the LED blinks on and off when the bottles interrupt the IR beam, then that means that there are spaces between the bottles on the belt and although your circuit  may seem to be working, the circuit is bogus and should look like the circuit shown below.

Also, the LED will turn OFF when the bottles break the beam.

If by "size" you mean the diameter of the bottles, in order to determine that you'll need to know the speed of the belt and the length of time the LED stays OFF as each bottle passes by.