Wicks come in different types and sizes. You will usually find small, medium and large (diameter measure) for each of the following types. Use one size higher for each 5 cm (2 inches) of candle diameter for long burning, drip reduced, non smoking candles. The relationship of the wick to the wax type and container or mould size/type is important for getting a long burning candle. Use a fatter wick for larger candles or for candles made from long burning wax, like beeswax or paraffin with hardening additives in it. When using flat braid wick, place the wick in your candles with the nap or grain of the braid down ('v' shape up). If you wick a candle with the grain of the braid going from bottom to top (the wrong way) when the candle burns it will develop a carbonized ball on it, and that will smoke and burn unevenly. Don't worry about wick direction with paper core or metal core wicking. It is important that you match the type of wick to the type of candle you are making:
Square Braid - Has a more sturdy structure. Use in moulded candles, container candles and dipped candles.
Metal Core - Zinc or Lead. Use for small container candles and votives or tea lights, tiny terra cotta pots.
Paper Core - Use for small container candles too. They may smoke more than a metal core wick.