These are what we now call 'big fucking cubes" but they are really all you need for making drinks at home. Depending on the size of your shaker/stirring glass, you'll only need 2 or 3 cubes (cracked with a spoon) to fill the glass. That is to say: 2 cocktails can be made with each tray.
That may seem excessive, but that is how drinks are made well.
To crack an ice cube, use an ice crackeror, just hold it in your hand and whack it with a spoon or a muddler. You will miss a few times, hit your hand, call me a fuck, get bruises, but eventually it will be easy and your drinks will be perfectly chilled and diluted.