[sc_rufctr]

I also ride a 53cm... Aside from using shorter cranks (165mm) etc there's not much you can do. My cranks are 170cm.
They do have slimmer seats that could help a bit but you need at least a "fist full of seat post" from the bottom of the seat to the frame.

The good new. You should be able to re sell the 55cm easily.

How are you measuring this frame?
(From the top of the seat tube to the center of the crank tube?)