How Can I Tell Frame Size?

I keep having the impression that my new bike is too big for me. The
vendor insists that for my 1.72 meters, the recommended frame size is 56
inches. A competing vendor suggests that a heavy person like myself may
be happier on one size down, a 53 inch frame, since that would make it
easier to keep a foot on the ground (I cannot start and stop or ride
standing).

However, I see no marking or designation on my bike whatsoever that
gives its frame size. It could be made for a 7 footer for all I know. Is
there any parameter I can measure to get an idea?

Many thanks,

Elisa Francesca Roselli
Ile de France