There are a lot of different variables to consider when deciding what size mini split will best suit your cooling or heating needs. a typical chart to reference is  350sqf to 400sqf =9,000 BTU's

                                             450sqf to 550sqf =12,000BTU's

                                             700sqf to 1000sqf =18,000 BTU's

                                        1,4000sqf to 1,500sqf =24,00 BTU's

                                        1,500sqf to 2,000sqf =30,000 BTU's

                                        2,000sqf to 2,500sqf =36,000 BTU's

of course  smaller rooms could require larger units depending on the heat load, larger rooms could require smaller units depending on the layout. that is why you should always consult license HVAC contractor before making your selection.