Ignore that dealer, he is dead wrong. The room's volume has ZERO to do with cone excursion. Only if the speaker is receiving less power from the amp, will the cone have less excursion. If you are actually playing the speaker quiter, but the SPL at the listening seat is similar to the higher power in the vary large room (because you're closer, and the room is smaller) only then will the cone move less. I build speakers...
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?