Choosing Between CB 6000 Sizes for Short-Term Wear

Hello! I’m interested in giving chastity a go with my girlfriend. We’re leaning towards the CB6000 but are unsure about sizing. My penis is 3.9 inches long when flaccid (not erect). Based on your experience, would you recommend the regular CB6000 (8.25cm) or the smaller version (6.35cm)? We’re planning for short-term wear.