You will need to make one and it will inevitably be curved in at least one direction.Find a good ash board big enough to make at least a couple and see what type suits you.Every user has preferences and some things depend on the height of the user and their body proportions.As a starting point find yourself a two foot rule as a guide to getting some numbers to work from and be prepared to glue wedges to the handle or plane them off until it feels right.The two foot rule is used by holding the 24 inch mark on the centre of the handle socket on the top face of the adze and swinging the zero end onto the front of the handle;mark this spot and swing the rule to the cutting edge and note the difference.I find that having the handle set so that the cutting edge is 5/8 of an inch closer to the mark than the centre of the handle socket is about right.If you start there and adjust until it feels comfortable,you will have a useful tool.It takes a few days of more or less constant use to get accustomed to the tool and to get a feel for how much wood to remove and in which direction.When you can split a match with it your are getting pretty handy.It is a fairly precise tool,not just a roughing tool.
This is very much up to you. Both straight and curved handles have their fans and detractors and additionally the angle of the head at the end can be varied to suit a particular way of working.
I was watching a carver's video a couple of weeks back where he went into a bit of detail on how he handles his axes and adzes for the work he does, I'll try to hunt down the link and post it here if I find it.
Thanks for the info. I think I will make one in some scrap wood to establish the shape then use that as a template to make one in ash.
Incidentally the adze has a square eye which I thought was a characteristic of British adzes, the French ones have round eyes. Sadly the marks are illegible.