If you segmented a single slice and the slice is so thick that the area of the side wall is not negligible (this should be very rare) then you can compute the circumference and cross-section area from the known volume, surface area, and slice thickness with a simple formula (because you have two unknowns cross_section_area and circumference and two equations: volume = cross_section_area * slice_thickness and surface_area = 2 * cross_section_area + circumference * slice_thickness).
Alternatively, you can also use Segment Cross-Section Area module as described above (this is mostly useful if you have segmented on many slices).
In the future, you can measure 2D surface area directly using Closed curve markups:
