I'm using Polk TC60i's for in ceiling's ( back surround) Cant find my scratch notes but my backer boxes for these are 16" x 16" x 10" deep. My side surround in walls are Polk 265 LS's , I believe those boxes were 1.2 cu meters (?). If you do a search for " backer boxes " in this forum I believe my old post will show up. I actually copied the 265 LS factory boxes ( they were discontinued, but if you found them online they were going for $200.00 a piece (!) ) These had chambered inner walls/ polyfill etc. Quite extreme. But my copy cats sound pretty good. Without backer boxes your sound will travel to cyber space, into insulation etc. Remember to caulk your box seams with acoustic sealant and even your speaker cable entry hole. Its worth setting up an account on the Polk site. There are some knowledgable heavy hitters there that can assist with your speaker models. ( Polk speaker forum)
Thee Rock n' Roll Theatre-coming soon.