The size is a 2.5 inch for the rear panel. Typical this type of driver is going to be full range. You can order the replacement panels with the speaker openings from Jeep and the OEM dealers should have them by now. The maximum size you can play with additional mods is up 4 inches.
Now since you have indicated that you are going to have an new off-spring to the family, the worse thing to do is get a rear pillar speaker that has a lot of high frequencies. The speaker is going to be near the new born's ear. You are better off in getting a coaxial speaker that you can tilt the tweeter downwards for the rear door. As of the tweeter stay away from the metal ones. Again, this is not good for the baby in the long run.
Depending on your setup, if are going with an aftermarket DSP, many of them now you can setup for the example the rear door as bandwidth type crossover. This way you limit the bass and the highs.
Congrads on the new one.
Best for now.
Laredo X, Trail Rated, QTii, Sele-Terrian, Quadra Lift, Off-Road Adventure Package ii, Nokian Rotiva AT, Tow, AFE CAI, DiabloSport inTune 1000-DCX, Custom Sound System ( HAT speakers, JLAudio , Pioneer , JBL MS-8 & RF 3.sixty.3 -- see the writeup link below for more details)