I think it also depends on what type of small group training you’re going to be doing. The needs for equipment are different depending on small group personal training vs. an equipment specific small group training (TRX, kettle bell, bosu, etc) or a sport specific small group training (running, agility, etc). You’ll need room for each person to move, plus equipment storage and usage.
Also, depending on the type of small group training, some groups require more individual attention than others. How many people are you comfortable supervising? How individualized will the training be?
I keep my small group personal training to 3-5. My post-natal groups can go as high as 15. I offer more hands on guidance with the smaller groups and the larger groups are still managable for me.
I always err on the side of starting with a smaller group and then adding members in, once you know you can handle the size of the group.