Most subcutaneous peptide injections use 27-31 gauge needles that are 1/2 inch (12.7mm) or 5/16 inch (8mm) in length. Higher gauge numbers mean thinner needles, which are less painful but may take longer to inject. The CDC recommends 5/8 inch needles for most adults for subcutaneous injections, though shorter needles work well for leaner individuals.
CDC Injection Guidelines