There are a few makers of "Dual-Skis". That being said, surface area is independant of friction. In other words a Dual-Ski is not faster than a Mono-Ski. There is also the older and most frequenty used "Bi-Ski". Bi-Ski rigs use a speicial ski that can only be used with the Bi-Ski sled. Some folks have mounted Bi-Ski bindings to "regular" skis and used a Bi-Ski rig in that way. Dual-Skis rigs and Mono-Ski rigs can run with any ski one wants as they use a "regular" ski binding. Most of the time those bindings are modified to not allow for release. Since there is no leg to break it is better for them to not release. Also if one is an independant sit-skier they don't want to drop a ski as it would be very hard for them to retreive the dropped ski.
Here is a link to a Mono-Ski going fast: http://youtu.be/JnE9fa99U7Q
Here is a link to see a Bi-Ski : http://youtu.be/rq2Q9AgatKk
Here is a link to see a Dual-Ski : http://youtu.be/xsguRUHdM-Q
Studies have shown that length, or size does not really matter if the preparation is the same. It's more a factor of weight and aerodynamics.
As for the cost of sit ski rigs, yes they are very spendy indeed. From around $3,500 to $6,500. The Bi-Ski rigs have no suspension, so they are the cheapest. Most expensive are the Dual-Ski rigs.
Most people who need a sit ski strive to learn a Mono-Ski. They might start out learning on a Bi-Ski, then progress to the Mono. Witch rig is best for an individual may also depend on their level of disability.