Anal plugs

So me and my boyfriend want to do anal . I got a at if like plugs a small medium and like large one . I’ve been on the medium for a bit but scared to move to the larger one and I don’t know when would be the right time to do so , medium is not painful to put it and isn’t majorly uncomfortable when in for periods of time , but I’m just unsure when would be the right time basically to move up.