For those mugs with the top wider than the bottom I traced a starbucks cup onto tar paper. Frist cut the bottom out of the starbucks cup, then cut down the seam, and trace.
There is a material called tyvek that works well or you can use Manila envelopes a few times. Sprinkle the Manila envelopes with cornstarch to keep the clay from sticking. Tar paper is also called roofing felt and can be bought in hardware stores in the USA. It is the most durable long term, I have used the same templates for ten years.
4"x10" is a good short diner mug, but you can go as tall as 6.5" x 10". I would not go narrower in length than 9" or the mug may bump the nose when drinking due to narrow diameter.
