What Size Needle for B12 Injection? Does size matter? Historically, a bigger needle gauge was considered better to prevent damage to red blood cells. However, our recent guidelines recommend using smaller gauge needles (2325 G) for all Purchase Needles (PrecisionGlide) 1 inch or 1 2 inch at BiosenseClinic BiosenseClinic CA VanishPoint 29 Gauge x 5 16 inch Retractable Insulin Syringe with Needle 1 mL, Regular Wall, 100 Box