How much is a part pertaining to measurement?

When a recipe calls for a "part" something, it comparable to a ratio. For example, if a drink recipe calls for 1 part sugar and 3 parts water, this means that the ratio of sugar to water is 1:3. If you put in 1 cup of sugar, you would need 3 cups of water. Two cups of sugar would need 6 cups of water, etc.

A "part" is just what it sounds like. It's a part of the final product. The size of the part varies and does not have it's own set measurement. It is only in relation to the measure of the other ingredients.