What is the minimum and maximum length of the 18-inch towel bar with extender?

This measurement is called “projection” and can be found in the images on our product pages at our retailer partner websites. Generally, square shaped pulls will have wider projections for larger hands or knuckles. Here's an example for the solid bar pull. 

Overall length is the total length of the pull. Center-to-center is the distance between screw holes on the pull body. The center-to-center distance is how you determine what size pull you need to swap out existing cabinet hardware. This information can usually be found in the image carousel (the collection of images on a product page). It might also be listed as product dimension length with other details such as finish and material. Lastly, you can find the overall length in the specifications section on the product page. 

To accommodate the needs of all our customers, we make both solid and hollow cabinet hardware. The Franklin Brass® Solid Bar drawer pulls are a popular choice for those preferring solid construction pulls.