I bought 1/4" shanks and the length is determined by the width of your collar that you put on your freezer.
I used a 2x6 to make a collar around the top of my freezer. I then placed a 1x8 piece of maple on the front and sides. The top of the 2x6 and 1x8 are flush with each other. The maple then allows for a support around front/sides of the freezer and you don't see the seam that the 2x6 makes. You have to drill through the 2x6 and 1x8 to insert the shank and that is a total of 2.25". I bought a 4" long faucet shank and it worked well.