This shape is very slim, and has a little facet around the edge. You can leave that facet crisp, and fire polish sparingly, or you can go a bit longer an fire polish the facets into a soft rounded edge, giving the soft sea glass feel to the edge.
The 2 shapes in the Sweetie Square Round Duo measure:
21mm x 7mm thick
Due to how slim the shape is, it is recommend that you use either 1/6th or 5/64th mandrels, and do not use a 3/32nd mandrel.