For length, just put the stock screws through the cover and look at how long the threads stick out past the mating surface - obvious if they should be used or not. The stock screw heads may not fit in those counter-bores either.
IMO, that cover is designed to be used with different screws or I would atleast get some better grade screws (stainless) of the same thread pitch, size and correct length, typically allen head cap screws would've came with something like that, since practically every aftermarket diff. cover manufacturer supplies their covers with cap screws for install most of the time.
I know Mag Hytec does, and I know TA-Girldes come with em'.