Power = voltage squared divided by resistance. You can increase power by increasing voltage or decreasing resistance, and vice versa.
The resistance of the coil depends first upon the inherent resistance of the wire you use. The larger the diameter of the wire (the smaller the gauge number), the lower the resistance, and vice versa. Material matters. For a given diameter, Nichrome 60 has lower resistance than Kanthal A1.
The length of the wire you use also determines resistance. The longer the wire, the higher the resistance, and vice versa. Translating that to a coil, the larger the diameter of the form the coil is wrapped on, and the greater the number of wraps, the higher the resistance, and vice versa.
How you apply that power from the coil to the wick (and hence the
juice) also matters. Using the above, you can see if you use less wire, for example, fewer wraps, you lower the resistance and increase the power. However, doing it this way means more heat applied to a smaller wick area, and may result in a hot, burned taste. The alternative is to use the same number of wraps of a lower gauge (thicker, lower resistance wire) to lower the resistance and increase the power while maintaining the same (or greater) contact with the wick. You don't want to overdo this; however, because the coil mass increases faster than the power. Too much can result in slower coil heat up. That's why it's better, for example, to use 26 AWG Nichrome rather than 25 AWG Kanthal, to go lower in resistance than 26 AWG Kanthal (we're talking high-power, sub-ohm resistance in this example, something you might want to have more experience before doing).
Making trial
coils of various resistance is fast, cheap, and easy. I put them in an ice cube tray on a piece of blue masking tape with appropo info written on it.
I find the 30 gauge wire a little easier for me to use. It's a little thinner and a little (tiny bit) more flexible. However....I think maybe it's not quite as "hearty" as the 28 gauge, so I might need to replace coils with it a little more often. But I don't care about that because I LIKE to make new coils anyway..... 
