IMO, I think the 8" would be fine.
Tennis balls, racket balls and any other small balls are a definite no-go, not allowed, toy in this house!
Ziltoid....DOGS DO CHOKE ON TENNIS BALLS!
I have seen it first hand. The get slimy from saliva and can get lodged in the throat easily. We had a K9 choke to death at a picnic when this happened...he was playing with the children.
EMS even tried to save him and a vet was on scene. He died despite every effort. It was horrific!
As with any toy, I always watch the dogs while playing...don't want them to break off a piece and choke, etc.
(I can't tell you how many toys, supposedly "non-destructible", have ended up in the trash can after 20 minutes.)
BTW, my dog LOVES the jolly ball! She becomes obsessed with it.
It's so bad, that I have to put it away and only allow her to play with it on special occasions.
FYI : Also, not a good idea to have them play with it in the house...that thing can fly across the room! ...at least in my case.