My carry-on, which meets but doesn't exceed the allowable limits, fits fine in the E170/175/190 family of aircraft. But it may be only on one side of the plane - though that may only be in F (going from memory from Monday's flight)

If it doesn't fit, or they run out of room, they won't plane-side check your bag (assuming that's what "valet" means - retrieving it at the exit upon arrival), they will check it to baggage claim. If you're in F, they might plane-side it.

These aircraft do run out of space quickly.