I guess you need to decide what type of projects you usually make. Do you use bulky yarn? or mostly worsted weight? That would determine the size of dpn's you would want to make.
You use the same size of double pointed needles as you would straight needles. For example if I am using a worsted weight yarn that needs a US 8 or 5mm straight needles, I would use a set of US 8 or 5mm double pointed needles. You can use a set of straight needles to measure the diameter so that you are making the correct sizes for your yarn. My husband has made me some out of dowel and we used an electric pencil sharpener to help point the ends of the smaller sized needles.Hope this helps. Good luck