Just look at this AMAZING wick database that matches wicks to the container and wax you are using.
It is HUGE!!!
Whether you are using soy or paraffin, container or pillar, thanks to CandleCauldron.com it's SO easy to find your wick.
Suppose I'm using the EcoSoya CB Advanced wax in a standard coffee mug container - using the search utility, I see that the database would recommend the LX 16 wick.
Now, obviously this is a recommendation and cannot guarantee perfect results, but hey! It is a great way to start!