There isn't a "sizing" methodology for Unleashed for a dedicated master.
The Unleashed Master AP doesn't need to be specced any differently than other APs in the Unleashed network.
Full details are in the Unleashed 200.9 release notes and the online documentation.
Hope that helps clarify,
To be pointed - the most recent release notes appear to express nothing in terms of this topic.
As far as the documentation it expresses the configuration option(s), but again - fails to answer the question.
FWIW - without a scientific metric to establish and validate need for something "more", would seem that any arbitrary [supported] unit in the line up would suffice. eg: Why wouldn't someone choose an R320 for purpose if the APs are a range of R3x0/R5x0/R7x0? Particularly, if each AP [itself] is handling the RADIUS calls and not being funneled through the dedicated master. In that scenario, the dedicated master appears to become purely an Administrative interface for configuration and pushing upgrades.
(Food for thought)