I have a 1995 camaro, they are small in the trunk, and I have the biggest possible box you can fit back there.
To find the volume of the box, just divide the box into seperate squares/triangles and add up the volume. But honestly, there's no way that your box is too big in a camaro for those type R's. A bigger box, especially only one slightly bigger won't cause your subs to be quieter.
You have two 12 inch type R's, the 122D's or the 1244D's?
You probably have two dual 4 ohm voice coil subs, if you have them hooked up right, you're only seeing about 300 watts rms on daily music to each sub. Those subs can take triple that daily with no problem.
Your problem isn't your box, it's your amp.