I made a very successful parm from 3 gallons in a 5" mould on 21st April 2013. I left it to dry for a month and then I started oiling with olive oil and ended up with a very nice parm that we started last month (and I just had some on my Spaghetti Bolognese for tea tonight!). That was 9 months old, I cut it halfway up and then cut one of the halves in half and used that and vac. packed the rest as it takes us a long time to use that much. I also have another on the go that I made 3 months later th exiling certainly gives a nice rind that seems very resistant to the moulds in my cave, blue just brushes off.
I'm certainly not an expert, I just follow the recipe, but this size certainly works okay.