Most coaxials are meant for IB installs in doors and such.Midbass output will be effected by too small. So I would say as big as you want, maybe even a little bigger. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gifIM making a box for skeakers that mounts on the wall its gunna be sealed for 6.5 coaxials does volume really matter .I was thinking .3- .5 cubes