I don't really know the average size. My biggest right now is right under 5 inches just shell length and mine are not too much older then yours as I remember the pictures you posted. Regardless, very happy to see two healthy turtles.
RES - Gator (6 inches) Male
RES - Petunia (5 inches) Female
DBT- Estobahn (5 inches) Male (adopted by a close friend)
CS- Crush (4 inches)(adopted by a close friend)
CDBT- Boss (2 inchs)
SawBackMAP-Bowser (1 inch)