It is 20cuft.
Sears Link
Sears used to sell a SV15 and an SV12, which were 15 and 12 cuft respectively. I (foolishly) bought mine new for MSRP-$10 when they were "on sale". Afterwards I noticed how frequently they showed up on Ebay and yard sales. Seems folks buy them and then either don't use 'em or are horrified @ the fuel milage they get w/ 'em on top. (Google milage and SV20 and see what I mean). After a buddy of mine saw mine (and liked it) he wanted one and I found one for him @ a yardsale for $25. I sold it to him for $75, so I hope he doesn't come across this post!
Coleman also makes one that is sold through Target. It isn't on EITHER of their websites, but every target that I've been into recently has it for sale. It is about half the price of the SV20 unit and probably half the quality. My understanding is that the SV20 is actually manufactured by Thule. (Perhaps the Coleman unit is too!)
The box is what it is. There's no getting around that it will kill your mileage, but for my needs... the ability to just take it off and get decent milage (as opposed to running a FULLSIZE van or Excursion) makes it ideal for me. Putting the thing on or taking it off is a 3 minute affair; it really isn't that tough. The most tedious thing about the box is finding somewhere to put it when it ISN'T on your Jeep.
- Jeff