Wasn't there an earlier thread about these massive beasts, particularly the sub-sub-contra-bass-how-low-can-you-go sax, otherwise known as the tubax? I have a CD that came free with a mag featuring said tubax, as well as saxes of all sizes, but predominantly the bigguns, and they are, well, texturally interesting to say the least.
Personally, I wouldn't want anything bigger than a bari as I too drive a Zafira.
Side note - I've found a good way of warming up to play alto is to have a blow on the tenor first; after the tenor, the alto feels all little and light and I can fair race around on it!
Cheers, Bob