I have used all 3 standard sizes successfully (S M L), and at time thought each of them were the best.
You (or your insurance) paid for a full mask KIT, which includes 3 pillows. They ripped you off. Not likely to be a mistake either, since it is a common practice among a significant number of DMEs.
Don't be afraid to try LARGER or SMALLER. Smaller fits more deeply but may not inflate enought to seal.
Larger makes a bigger 'plug' but may fold or just not fit deeply enough.
A P10 should offer NO noticeable leak, neither by FEEL nor by SOUND.
The only air escaping should be the 'ooze' out the front exhaust vents.
Do not be insulted if I mention that many people have turned the thing around backwards. The pillows should "v" in the same direction as you nose slants -- facing forward: /\
Seriously, it is POSSIBLE to wear the thing backwards without terrible leaks and without noticing for a while.
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."