When a recipe calls for a specific number of pounds of a vegetable, such as cucumber, how do you know how many to use if you don't have a kitchen scale to weigh them?
Look around in your cookbooks and you can often find equivalents. Many recipes include directions such as "1 pound tomatoes (X number of medium)."
Or go to freshpreserving.com, the site for Ball canning supplies. Look under "preserving guide" and you can download a very handy produce purchase guide that includes equivalents for most common fruits and vegetables.