Absolutdrinks.com makes measurements in parts. A part is simply a relative measurement, and works for single drinks and large batches. For example, when the recipe calls for 1 part of ingredient A, and 2 parts of ingredient B, you use twice as much of ingredient B.
Dash or Splash
A dash is bigger than a splash, but both are small amounts. You use them to flavor a cocktail for taste, so it's a bit of a judgment call. A splash generally involves less viscous liquids such as mixers, whereas dashes are used with stronger flavourings, such as tabasco.