Does the size of your mini split heat pump matter , better safe than sorry when you purchase your heat pump right , not always the case , size the mini split for the area that you are to installing , if heat pump is oversized you risk short cycling of unit, and possible defrosting issues in winter.
Short cycling of heat pump is happens when , the size of the mini split is to large it will heat up the immediate area quickly , causing the heat pump to cycle on/off regularly which losses efficiency , most mini split heat pumps need to run for 10-15 minutes in winter to active defrost mode which is crucial for preventing ice build up in exterior unit.
Best advice is to buy a good quality heat pump have it professionally installed ,yet just as important to size the mini spilt correctly for long term benefit through your home.