Conveyor belt project - proximity sensor

Hello, first time posting here
I'm looking to do a project for my 2nd yr exams and its a conveyor belt system that detects different size boxes as they pass by on the belt
Depending on the size of the box which is to be determined by a sensor, it will activate a solenoid to a pneumatic cylinder to push the box off the conveyor into a collection box
I need to find a sensor that works on a light beam breaking when the boxes of various sizes pass through

the sensors will be mounted on a bracket off the side of the conveyor

I'll be using this site

If anyone could help thanks

You would need a retro-reflective sensor, there are a few manuf. Omron is one.
This just requires the sensor on one side and a circular disc reflector on the opposite side.
Max.

Welcome to ETO!
Depending how you define 'size', unless the boxes all have the same shape and orientation you may need more than one sensor.