Hi Nick!
The bottles will definitely be 1 liter at the competition!
As far as sinkers, the rules state that there might be "different types of hooked sinkers (e.g. different weight sinkers with the hook located in different positions)"
The sinkers will all obviously have to be able to fit through the opening of a bottle, which limits their size, but could be really made out of anything small that sinks to the bottom of the bottle with some sort of hook or loop available for divers to grab onto.
For your own School Olympics, I would suggest constructing sinkers of various masses/shapes/configurations using any materials you have on hand that might work to achieve that.
We made a few revisions to the rules for the divers to make a few things a little more clear, so check them out again if you have not!!