The sizing of rosebud cylinders is more about getting a seal against your body rather than how big a dildo you can take. Where there is a significant crack in your as cheeks or where someone has a particularly pert ass, they may not be able to get a larger cylinder to fit against them I suggest using an appropriately sized circular object (but not glass!) to demonstrate this.
Have you seen the advice on the rosebud page? http:thickwall.co.uk/shop/rosebud-pump/rosebud-cylinder/
As for the probe, some people say that the probe helps create the rosebud an others say the exact opposite. For me personally, I just like the feeling of the probe which gets my ass in the right mood. So, I tend to think is a personal preference thing!
Have a read of the rosebud page and come back if you need more advice. If you are unsure, go for a smaller size as the 2.25” tends to fit almost everyone and the 2.5” fits most people.
