The words "An ye harm none, do as ye will" are appropriate for many places on the body. Common areas include the wrist, the back of the neck and near the heart.
Check with a tattoo artist regarding the final size because it may change depending on the script used for the letters and any additional elements to the tattoo, such as pictures or symbols.
When looking to get the full poems tattooed on your body, which are generally over 300 words, the back and the outer thigh are commonly used. Getting the Wiccan Rede tattooed on your body does not break Wiccan rules, but it may not be universally accepted by other Wiccans.