A coil is a length of wire wrapped around a jig a number of times.
Given the same wire the more wraps the higher the resistance and obviously less wraps means less resistance.
Surface area is how much wire is touching the wick. The coil vaporizes the juice off the wick so more surface area equals more vapor which also equates to more flavor. But there are more variables to surface area.
Which brings me to different wire gauges and resistances. If you stick with the same wire and want more surface area you will add more wraps and the resistance will go up which is why people go for a lower gauge wire to get more surface area and keep the resistance down. If the voltage stays the same and the resistance declines then the wattage will rise.
Man I hate that ecigs are so....hard to explain because there is much much more to say but I don't want to confuse anyone more than I have already.