Hi Frank and welcome to the forums. There's a principle called ohm's law which links voltage, current and resistance together. The easiest way for me to remember from my electronics days is as a triangle with V at the top and I and R next to each other at the bottom. It allows you to calculate the missing variable. In our case current (I). This is needed to work out power in watts. The more power, the more heat the coil will generate, same as a lightbulb (eg 40w vs 100w).
V
I R
You can use multiplication and division to work out the missing variable. so to find I = V/R to find V = I X R
Power = V X I
Using this principle here are some examples of carto resistances and their corresponding theoretical power basing it on a standard 3.7 device.
V____I____R___Power (Watts)
3.7__2.5__1.5__9.13
3.7__1.9__2.0__6.85
3.7__1.2__3.0__4.56
Most devices regulate voltage and try to keep it a constant, this means that because your resistance is fixed as well the only variable is current. Now a small battery will have a hard time outputting lots of current, and it will drain the battery quicker too. It can even damage the battery if the resistance is too low, hence why people recommend you don't use Low resistance (LR) cartos or atties on some devices. Some of the larger batteries are better able to output high current and they can also maintain a constant voltage for longer and more consistently over the discharge period. Its this consistency that many of us look for so that we can dial in a voltage and then not have to worry about it again.
Because everyone has their own 'sweet spots' for vaping together with all the different kit which varies from the theoretical values, what works for you might not suit someone else. Because you can vary your voltage you have much more control. As deemer has suggested, picking a resistance somewhere around 2-3 ohms will give you lots of scope to experiment.
