We came across a case this week that demonstrated the most likely scenario we would expect to see on Day 5/6 at the Blastocyst stage: a grade 3, 4, or 5 Blast. As the embryo increases in cell number, it also expands physically. The goal of expansion is to increase in size so the surrounding shell (the "zona") stretches until it breaks. The embryo can then hatch so that it is able to implant.
The above embryos were hatched artificially on Day 3 so our lab could biopsy them. You can clearly see the size difference and compare the 3 stages of embryo expansion between them. A grade 6 embryo would mean it has hatched out. A grade 3 embryo is the exact size of the shell.