get huntin for deals on tinterweb, i could help you with home audio and quality gear such as bose or tannoy etc, but as for the bottom end gear that you get from pc world or currys for your pc then i wouldnt have a clue as they are all unknown makes and never around long enuf.
would hunt for what you need on the net then and then look at the specs and they should give you a size of box for the sub and if they dont...then look for the size of speaker in the sub and then work out the volumemetric size of its box.