"Elisa Francesca Roselli" wrote:
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).

I think you mean 56 and 53 cm. Frame size is specified by seat tube length.
Different manufacturers use different methods of making this measurement. It
is always measured FROM the center of the bottom bracket. It is measured
along the seat tube TO either the top of the seat tube, the top of the top
tube, or the center of the top tube. If the frame has a sloping top tube, it
may be measured to an imaginary point where the top tube would intersect the
seat tube IF it were level. As you can see, it's not easy to compare sizes
between different bikes and especially different manufacturers.

In addition, there's more to "frame size" than just the seat tube length.
Top tube length is probably more important as it will determine how
stretched out you are on the bike.

Height is not a very accurate way to choose a frame size. For determining
seat tube length, your leg length or "inseam" is the critical parameter. Top
tube length should be chosen based on torso and arm length (as well as
personal preferences and type of riding).

If the saddle seems too high on the 56 cm frame, maybe you can lower it a
little. Then when you become comfortable starting and stopping, you could
raise it up. Having the saddle too low doesn't allow you to extend your leg
properly. That can limit your power and can cause knee pain.

http://www.sbraweb.org/setup.htm

Art Harris