Most of my pipes are 3/4" and 7/8" bore with chamber depths of about 1 7/8". I consider anything smaller a small pipe. While I am not very familiar with Stanwell and Savinelli sizes, I do know about Peterson's. I would suggest a 303, 314 or 317 in Peterson's System pipes or a 150, 408 or 221 in their regular pipes.
It would be hard for one to go wrong with any of the three I mentioned.
The group sizes you mentioned are a Dunhill reference as Searock mentioned with a group 1 being a very small pipe, a group 4 an average sized and the ODA large pipes.