The three variables that have to be considered in sizing and selecting the unit include:
Determine Desired Temperature Rise
The difference between the the temperature of the hot water exiting the heater and the cold water entering the unit is called the temperature rise. If you want a shower up to 110°F and you live in south Florida with groundwater at 72°F, then you need a 38°F temperature rise (110-72=38).
A tankless water heater is sized by rating its temperature rise at a given GPM. So a unit could be rated at a 33°F Temperature Rise at 2.0 GPM. Based on manufacturer's data, this same unit could also provide a 65°F Temperature Rise at 1.0 GPM.
You see, the slower the flow of water through the unit, the more the water can be heated.