As someone who is 5'6" tall, you should be able to fit comfortably on a regular twin adjustable bed (only true when in the flat position) which typically measures 38 inches wide by 75 inches long. However, if you prefer extra legroom or if you tend to move around a lot in your sleep, a twin XL adjustable bed might be a better option. A twin XL bed is 5 inches longer than a regular twin (this five extra inches makes all the difference when you have your bed in the heads up position which effectively shrinks the length of an adjustable bed by pushing your whole body towards the toe of your bed), measuring 38 inches wide by 80 inches long, and can accommodate taller individuals. Ultimately, the decision between a regular twin and a twin XL bed comes down to personal preference ( I guess it is true some people will not mind the squishing) and what feels most comfortable for you.
(I recommend visiting or Mississauga showroom to for an in person experience to find out for yourself)