There are a few types of sit skis, including mono ski, dual ski and bi-ski. The mono ski gives the greatest amount of freedom and performance, the bi-ski allows for people with less balance or strength to ski and make turns using only lateral movements.
When setting someone up in a sit ski there are options of different seat sizes and angles, footrest length, ski mounting to find a centred position fore/aft, and outriggers, and skis themselves.
For mono skis an ideal ski has a length the same that you would use for the person’s height if they were stand up, it’ll have a good rocker and be an all mountain ski width to encourage rotation, and be built tough. The bindings will have race din settings and/or a lock out to prevent them releasing.