I use 10mm gravel, although if you intend to free-loft it doesnt really matter what size the gravel is, If tethered then the larger gravel would quickly blunt talons if the bird was batey.
If I was you I'd make provision in your aviary design to be able to tether the hawk, if it is a juvenile HH you will definately need to tether it, at least during initial training....and probably for a good while thereafter.