One thing to target is an adequate cycle time, especially for applications where dehumidification is needed.
If the mini-split is oversized it will short cycle meaning it will quickly cool the area and shut off which doesn't circulate a lot of air. Short cycling doesn't allow for much dehumidification.
I recently sized and installed a mini-split for a 600 sq ft basement, several of the online calculators suggested large tonnage, others smaller. I went with the smallest mini-split in the productline that I was interested in which is 9000BTU. It is more than adequate at pulling down the temps and it does pull a lot of moisture out of the air. The condensation line flows quite a lot during operation. An added benefit with going smaller is the smaller air handlers are quieter.
the thing is are you in a heating or cooling climate.the minis are inverter compressors (most are).normally you would size for the cooling load and back up heat to pick up from balance point>are you doing cooling only?no back up heat in a mini so i size for 80 to 90 % of the heating load since it unloads for cooling .dont know what part of the country you are in however
its for a small summer house. we are in Turkey...Agean area. winters are mild. usually hovers above freezing. summers are 90-100. not desert dry and not swamp humid. will use it mostly in summer. will need heat since family lives there year round.
Not sure how to figure out r factors. I'm not familiar with home material names. But the average construction is very different here. All concrete and brick. Wall are not very thick. Dont know how to describe the bricks..... they are very light and has a kinda honey comb structure. No attic to speak of. It's like two pieces of corogated tin with about 3inches of foam in the middle. Assume they are going to put a kinda Spanish tile like cover.