At 5.5" she was likely about 4 weeks old at the time unless she was malnourished, resulting in being a bit behind size-wise.
At a few months old you should be able to get a good idea of her gender. Beardies of both genders will wave, but this is often done by the female dragons when they're older. All my beardies waved as babies, and at about 6mo old my male became a macho man and started the head-bobs while my female continues the waves and slow head nods.
There's a good guide on this site about checking gender, but basically you'll want the dragon flat on their belly, and you lift their tail up to about a 90 degree angle. This will cause 1 or 2 different set up bumps to show near the vent. One center bump means female, 2 bumps at the sides means male.
If you cannot see any bumps doing this, you can also shine a flashlight near the base of their tail while it's up and the bumps will show up as dark masses