Smallest over flow catch???

The top hose acts as a vent/overflow for the catch catch can - nothing else. In the pic the end is just tucked out of the way.

For a recirculating system to work you need to have a recovery cap on the radiator. The only difference between a recovery and non-recovery cap is an extra rubber seal on the cap that seals against the radiator neck.

Modern cars use a recirculating system. Old, old cars just puked it out on the ground. A non recirculating can just collects boil-over and you have to manually drain them. Here, if the radiator pukes, it travels down the hose and into the can at the bottom. Later, when the system cools down, vacuum in the radiator draws the fluid back out of the can.