Thanks to Mike making the following comment:

Mike wrote:
I am new to candle making and was wondering just how do you determine the jar size? Do you measure the top opening of the jar or the widest point of the jar?

When determining the diameter of your jar you want to make sure that you will be able to get a full melt pool at the widest point of the jar. Therefore you should use a wick which will be able to burn the diameter of the widest point. All of our recommendation charts have a wide range of recommendations for each jar diameter. We recommend starting somewhere in the middle of these recommendations and then testing larger or smaller wicks as needed.

Hope this helps!

Jessica