I agree. One size doesn't fit all....I suggest that you try a pump up type if in doubt. And of course, a gental warm up always helps. I have met several providers that love a tight fitting butt plug. Some will not even consider fucking without one.
When to stop pumping?
Trial and error. I usually take about 5 full squeezes, but I'd guess YMMV. You can vary the speed also, and varying the pumps is also good. Surprisingly, it often feels good to deflate and turn the vibrate on high just bevels splashdown. The sphincter pinching down and the vibe on high can have an interesting effect on the prostate. The pump doesn't really stretch the sphincter much, but you can feel it inside and the more you pump, the less likely you are to expel.
Geez Elizabeth, you ever travel to Texas?
Sorry, before splashdown. iPhone.