Thanks for this! As Marisa was kind enough to point out, though, I was measuring width and not true "size" - carapace length. Regauging, now, I think our little girl is gravid

She really wants to get out, and digs like mad in the pile of rocks we gave her for basking.
And that is indeed how I found out which one was male and female - nail length & tail size. I was told the incorrect information about their gender by the person we got them from (that's one reason I am happy we got the turtles away from their previous owner . . . she had A LOT of incorrect information!).
Oddly, though, I wonder if our "male" is actually both genders. These little guys are supposedly the same age, yet the female is the smaller, and the male, with long nails and a wide tail, is at least 1.5 inches larger! I know it can happen, based on incubation temperature, but we have a vet appointment coming up and I guess I will let our vet tell me the real deal!
I am going to get some nesting materials later today. If nothing else, it won't hurt. I appreciate your input, though! It really makes me feel like I am learning something new! I have rescued many animals, but RES are a first. It all seems to be falling in place!
(Incidentally, for other watchers who posted before, we are getting a 55 gallon tank. We still might separate them, though, so we are holding on to the 20L)
Again, thanks, everyone!