Your question is somewhat confusing, but if I have interpreted it correctly, your dealer is wrong. A smaller room should attain higher volume levels than a larger room for the same power input. Also cone excursion is independent of room volume and is only a factor of power input. Hope this helps.
Does room size affect volume and bass?
I listened to a pair of Sonus Faber Electa Amatour II, I love them, only one problem, when I have one of my Reference Recordings with the Dallas Wind Symphony on, when it is playing loud, like around 90 DB and the bass drum cracks the speaker cone reaches excusion. This was at my dealers room which is 4 times the volume of my 19x13x8 room. He said that if the speakers were in my room playing at the same volume, they probably wouldn't hit the cone excersion. What do you think?