The size is a minimum of 38 ftt (12 meters).
However, the validity size will be listed on your SLC depending on the size of the vessels on which you have experience.
The formula is to add 10 ft (3 meters) to the size of the vessel you have often sailed and is listed in your logbook.
For example, you have a 30 ft boat at home and have not sailed a larger vessel. Your SLC will be valid for 40 ft.
The same rule applies for catamarans, however, you must have the catamaran qualification or catamaran endorsement