Buying pre built coils???

[VapingBad]

The main principles are more surface-area/contact-area (contact between the coil and wick) the more vapour, needing more power, bulky coils take longer to get hot, so there is a trade of there. But more importantly having coils designed to be ran at more twice the power you use won't be good, neither will having them too small. Resistance only really matters in how well your mod will power them, it is all about the physical size and heat, airflow and wicking. Like a larger heating ring on an electric cooker will cook more food than a smaller one, nobody cares what the resistance is. Thicker wire gives more contact area, twisted and Clapton wires even more, there is no substitute for a bit of experimentation to home in on what suits you, take suggestions from folks vaping at a similar power level as you for a good starting point and ignore the ohms (as long as they are in the range for your mod).