I am a guy and currently play with a butt plug that are have a diameter of 35 mm. The sphinter is only streched to that temporarily and it fits with a bit of patience. The dildo we use for pegging has a diameter of 28 mm.
The one I think about buying now (https://www.funfactory.com/de/dildos/magnum/) has a diameter of 38 mm and it is 18 cm long. Can I play with that (carefully of course) without experience negative long term effects? I think that the sphinter will eventually become too streched to stay closed then not playing, and I certainly do not want that.
There's no definite answer to your question as everyone is different. The sphincter is a muscle and like all muscles you can train it to become stronger and more flexible, and like all muscles it will strain or tear if you overdo it.
so, warm up, use lube, listen to your body and stay healthy and there's no reason you can't keep going up in size gradually. Your body will tell you when you've reached your limit.
If you do feel intense pain there it usually means you've ripped the muscle slightly and you should give it a good two weeks to recover.
Every one is different but as a rough guide 65mm dia. 2.5" should be fine, it's once you've got to 3" or 4" that you may notice some slight change( more in look than function ) if you slowly increase the size you'll be fine and you'll be amazed how far you can go if you wish, as sum sub said your body will tell you if you need to slow down or give it a break for a few days.
A diameter of 65 mm is a good number to keep in mind, then. Thanks! I currently think that I do not want to go do arbitrary size, I just want to have a sensation that is slightly more filling. I guess then 38 mm should be just fine.
38mm will be fine. You shouldn't have any long term effects. As said above, the inflatable butt plug will make you feel full. Once inserted, you just pump it up.you'll be suprised how big you can get it. It is a very strange ( in a good way ) sensation. It also vibrates. The actual plug itself isn't that big, its rock solid though. The only down side is all the wiring.
Your sphincter should be fine, it's a muscle that gets worked out every day. I've been fisted in my ass during sessions and it still goes back to normal. It only stays open for a few hours before shrinking back down to close.
On the plug side, I have one with 35 mm maximum width and it gives me sufficient fullness. Currently, I would like to try more during pegging, and there an inflatable plug does not apply.
@Sarahgee: I believe that the muscle can tighten again after a short time. What concerns me more is that it will eventually loose tension in decades, and I do not want to deal with that earlier than really needed.
I don't actually know the size of my favourite butt plug but it is certainly much bigger than I ever thought i would be able to take when i started. It is still a bit of a struggle to get it in but it feels just great when it is and I haven't had any problem with tearing. As regards fisting I have now ben fisted through the third 'gate', again something I never thought I would achieve. I was worried initially that I would have indigestion after wards but so far no problems. I am now bent on getting fisted up to the elbow! Any warnings?