You can use pickling or regular cucumbers for this dill relish. Since you are chopping them up, the size does not matter. One consideration is that you will need to remove any seeds. Smaller pickling cucumbers will have few, if any seeds, so they will make things easier. If you don't have any, regular cucumbers will work fine--you'll just have to de-seed them. Whatever you choose, make sure it's unwaxed.