Select the size by determining the best seal or picking area - ideally you are seeking a smooth surface to achieve a good seal. If you are not sure of the lifting capacity of the different cups you can use the Lifting Forces chart on this site.
The larger the cup the more lifting force will be available. If however, a single cup cannot be used consider multiple mountings of smaller cups on a manifold block.
Select your preferred vacuum cup style in the vacuum cup catalogue. For example, if the sealing area is not smooth, consider a featheredge cup (e.g. 2E) and if a pulling motion or immediate lift is required consider a bellow style cup (e.g.33A-2 Long).
Find the matching or corresponding diameter as illustrated in the detailed specifications.