Your dealership probably doesn't know what he is talking about... Here is the search results from Tire Rack.
For 18" rims.
Bridgestone*Blizzak LM-60 Coupe&autoModClar=Without Sport Package&wtpackage=true
For 19" rims.
Bridgestone*Blizzak LM-60 Coupe Touring&autoModClar=Sport Package&wtpackage=true
Notice the price difference? 18"s are much cheaper. And yes get any 18" you like as long as it clears your BBK and not too wide.