Ice melts. That's what it does. Big ice melts slower than small ice. Drinks that call for big ice benefit so much from big ice. Please, please do not think that this is optional. Ice makes a huge difference in cocktails.
During the lifespan of a drink, the ice melt changes the character of the drink - sometimes to an extreme degree. Drinks that use big ice generally do not appreciate a lot of extra water content, and 20-minute old version will not resemble the fresh version.
The classic way to make big ice is to start with a huge block of ice, and chisel out a piece that is slightly smaller than the glass that you'll be using. This is wonderful, and beautiful - it is also painful and time consuming.
I've found a very reasonable and simple way to make big ice is to fill a 12 ounce plastic cup not quite halfway, and freeze it. After it is frozen, you can cip the edges off with a barspoon.
Cocktail Kingdom also sells a 2 inch cube ice tray.