Depends on several factors. For example, I have a 12x8 sealed tuff shed that is insulated with foam everywhere but the floor and it stays with in optimal temperatures in the summer in Phoenix, Arizona with a mr cool 12k btu mini split. I’m running 2 680 watt growers choice lights, one dri eaz dehumidifier, and a water chiller that produce heat as well.
24000 for sure. You'd rather your compressor cycle/turn on & off,than run constantly.Most of the minis dehumidify quite a bit too. Better too much than not enough. For a few hundred bucks more,I'd go too much.