I've not seen research resulting in empirical evidence for small class sizes for gifted pupils, however research does suggest that cluster grouping by ability is beneficial.

There was one reference to this in a recent post on another thread.

With gifted kids being a small percentage of the population, cluster grouping by ability and readiness to learn at the appropriate high level of academic placement and pacing translates to small class sizes. Unless the classes are "filled in" with other students, as they often may be, to the detriment of student learning.