Gotta be 100mm, i.e, 4" min for revolvers and 120mm which is about 4.75" for autos. I was looking for 4, 5/8" originally but never found any, ended up settling on 5.5" as that was all that was avalable and have grown to like them. Mine are all standard (apart from a poor man's trigger job) so far. I have a 5.5" NMSBH and a 44 old vaquero. The 5.5 is surplus to requirements and I can't seem to find another 44Vaq to match the vaq I have so I am considering vaquero'ising it.
PS Daks, where in Australia are you from, I'm from Bathurst, NSW, shoot with the Cudgegong Rangers at the Mudgee club.
Mick