First things first, start by checking your remote.Mostly if your AC is not cooling, the units need service, cleaned, and then checked for gas pressure.
If Gas pressure is low then, Gas top-up needs to be done.
Reason 2 Another reason for your AC not cooling could be a bad or defective capacitor. The compressor will not start if the capacitor is defective
Reason 3 Problem with the outdoor unit's Fan Motor.
The AC tonnage required depends on personal preference.
For moderate cooling 1 Ton of AC for 150 Sft room for residential is applicable.
Depending upon the Heat Lead by Commercial Application Tons required shall be calculated.
As per the Thumb Rule,
1 Ton AC is required to cool 130 - 160 sft area zone. The temperature to be maintained for human comfort is 22*C to 24*C.