Are you aroused enough? If you’re partner is bigger you may need to wait until you’re very aroused, mentally and physically, before moving to penetration. Adding more clitoral stimulation (or wherever you know you’re most easily aroused) or using a vibrator can be a fun and effective way to make sure your body is as ready as your mind is.
Add lube If you’re having intercourse with someone with a big penis using extra lube is always recommended
Recommended sex positions The best sex position will be one that allows both of you control over the depth and angle of penetration and offers as much movement as possible. Avoid positions that allow for deep penetration (e.g., rear entry sex position) and favor positions that give you more control. There’s no easy answer here, since something like the woman on top position does allow you to control the depth of penetration, but doing so becomes a workout for your legs. The missionary position gives the person doing the penetrating control over the depth of thrusting, but if you’re the one on the bottom there’s no much you can do. The key is to experiment until you find a few that work well.
or even a ring around his penis limits the length that will enter the vagina during intercourse