The smallest is th Chemspec. Small enough to get through all doorways and not too heavy. My largest one is the Karcher 66. With twin vacuums and twin 66cms contra-rotating brushes it is incredibly efficient in soil extraction and with such a wide track is a great time saver.
The downsides are that it is wider than the doorways of my house and very heavy. It has sometimes taken me 30 minutes to get it from storeroom to van and then I need a short rest and the right technique to load the thing. But on one regular job it not only does a better job but saves about 90 minutes cleaning time.
I think that the decline in their use is largely due to cleaners feeling that the increased power available from electric extraction machines has rendered them redundant so the high cost cannot be justified.
On the other hand to those of us who wish to produce the best results possible rather than just an acceptable one, they are worth the additional expense.
The JAGUAR MX-2 ETM @ cross-american.com