Ahh, the anti flatulator! Got it! Thanks buddy!
If they all "kinda fit" -- try each one for 3 nights to see which gives best numbers, is most comfortable and causes the LEAST irritation (or hopefully NONE.) It usually takes a couple of nights for minor irritation to develop if the pillows aren't terrible but also aren't good enough.
Occasionally at first, revisit the "other sizes" unless you are TOTALLY DELIGHTED with the ones you pick.
Retired_guy is correct, the main thing is the tongue position -- I am very fortunate in that the back of my tongue naturally rises up to block the back of my mouth. In fact it takes a significant effort for me to let air in the nose and blow back out the mouth.
At first, I concentrated on holding the ENTIRE tongue against the roof of the mouth while keeping the lower jaw closed and pressing the tongue upwards.
Another (almost psychological) way to approach it is to try to remove all air space from your mouth. If you can do this, you can ALMOST set up a suction that glues your mouth shut. Note this is almost more of a mental effort than a purely physical trick.
At this point, I can open my mouth fully without allowing any air to escape, but it took a LITTLE practice to get there.
Some people with SIGNIFICANT mask leaks have reported wearing a chin strap for years and then discovering that it had trained them to keep their mouth shut and the strap was no longer needed.
Also, for the most part, a chin strap is FAR more comfortable and convenient than a FFM and the associated gear and fitting.
For all practical purpose, when using a FFM (or similar) my chin and mouth are forced into very limited positions that CAN work. I never even figured out how to breath through BOTH nose and mouth with an FFM, and had to 'set my jaw' for one or the other.
Sleep study AHI: 49 RDI: 60 -- APAP 10-11 w/AHI: 1.5 avg for 7-days (up due likely to hip replacement recovery)
"We can all breathe together or we will all suffocate alone."