Wicking is probably the most difficult part of making a candle---anybody can make a candle, but not everybody will make a GOOD candle. It is not because they can't, it just takes some perseverance (time and money also) to test, test, test.
This is what sets a good candle apart from the rest. When you hear that a candle burned all the wax out of the edge of the container, it wasn't the good wax, it was the good candle maker!  And when you see a candle that doesn't burn all the way to the edge, it isn't the fault of the wax, it is a poorly made candle, that is all there is too it, plain and simple! There are rumors out there on the web that soy wax tunnels, it doesn't tunnel, it does require a larger wick than paraffin usually, but the improper wick size is the cause of the tunneling, not the wax).

Unfortunately, the way to a good candle isn't easy or effortless-it isn't hard, but it takes time and some $$$. It requires testing to find the perfect wick, and wicking depends on your wax, your fragrance oil, dye, container (both what the container is made out of and the shape!). Even altitude will affect the way that your candle burns. You may have to try 6 or 8 or more wicks before you find the perfect one for every combination of wax, fragrance oil and dye. Many candle makers will test 5 or 6 wicks in each container with each different fragrance before deciding which wick is perfect with that particular combination.

The following is a starting point: All wicking is determined by the DIAMETER of the container. The height of your container only determines your wick length, but the wick size is determined by the container at its widest width. We recommend that you first decide the containers that you would like to use. Then choose the wax. Then the fragrance oil. Try about 6-8 different wicks in the range for your container. KEEP GOOD NOTES!!! With practice, you'll be able to have a pretty good idea as what should work. 

Our Best Wick Testing Advice: For each different candle container, fragrance and color combination you plan to make, make 6-8 of the exact same candles using a different wick size in each one. Label the container with it's specifics and wick sizes. Place near each other on a table, put a sticky note in front of each candle with the wick size clearly visible and take a digital photo of the group every hour as the candles burn during testing. Do this through the entire burn process. Check for a Good Melt Pool. When you have burned your candle one hour for every inch that your container is across, at it's widest point, (so if your container has a 2 1/2" opening but it is 3" across at it's widest, you should burn your candle for 3 hours to check the melt pool), your candle should have about a 1/2" melt pool. If you don't have a 1/2" melt pool, your wick is too small. If you have more than a 3/4" melt pool, your wick is too large. The pictures are great because you can go back and look at your results later. Don't make your final decision until you see how the wicks perform all the way to the last 1/2" of the bottom.