If you multiply 27 x 27 x 4.5 (depth) x 5.9, that comes out to about 19,300 gallons. Depending on your climate and pool use, you would benefit most from a system with a maximum stated capacity in the 30,000 to 40,000 gallong range.
More info about system sizing: https://discountsaltpool.com/What-Size-Salt-Pool-System-Is-Right-For-Me