You should learn what all the Dunhill group sizes are as they are an excellent way to learn how big or small a pipe is. The group sizes are guides, they are not exact but they are very useful. If you are on ebay looking for a good size pipe like a group 5 size, once you know the sizes you will just pass by all the pipes that don't fit what you are looking for.
The group sizes are for the bowl size, not the length or weight of a pipe.
A standard group 5 is around the following dimensions.
Bowl height- 1.86-2.15"
Bowl depth 1.35-1.75"
Inside bowl width .75 - 1.00"
You can get a large group 4 and one guy will call it a small group 5, and both guys can be right. As I said, they are a guide and not an exact thing.
Dunhill group sizes go from group 1-6 and the largest is called an ODA.
I hope this helps you.