I prefer plastic dumbbells for my kids, 'cause they tend to mouth with the wood ones (comes from a stick chewing tendency in puppyhood I think). Wooden ones tend to bounce less in the ring however. Wood ones though *may* break though. (had that happen to a friend when she was in the Open ring. The dog brought back both pieces!) For the most part, it's whatever works for you and your dog.
This is an *o.k.* fit on Hoss. The bell sides are flared, so it is comfy for him. Dumbbells with straight bell sides are harder to fit for him 'cause he's got a big face. You can't see it very well from this pict, but the bit/dowel is again an *o.k.* fit, as the canines just barely over lap each other.
The other thing to consider is if the bells are large enough. The dogs nose should come just to the edge of the bells, so that he/she does not bump the nose against the floor as they pick up the dumbbell. For labs some of this may be irrelevant, 'cause of their enthusiasm in retrieving.