What is the best width for the black curtaining (legs) down the side of the stage. I am informed 36" to 60" any suggestions please before I buy the fabric.

Really depends on the venue, I suppose.

The position of the auditorium seating and distance between the legs would be your main deciding factors.

I would actually tend to go a little wider, perhaps up to 6' (you can always double the end over, or leave some fullness). If you use 4' legs and the cast rush past them, they might billow and the wing might be revealed. I tend to prefer chain weighting them. Whatever you do, avoid semi-hard blacks. I have seen these in use and I think they are lethal. The on-stage edge is stiffened with a wooden batten and I was just waiting for a cast member to rush off stage straight into them. Fortunately, it never happened, but I hate them.

Hope this helps

Dave

As suggested by DaveB get the material as wide as available & your budget will stand. It always far easier to reef the leg drop on it?s self, than trying to extend it with a goose wing leg drop.
As every space is different & is dependant upon sight lines, as posted by Pritch. So move around the extremes of the auditorium & get someone to markout the sight/view lines with gaffer or chalk, which will determine the width you need. You can also do it with someone holding & moving a jogger flat, which may give you a better idea.
If you can afford it I?d go for wool fabric, to be made up with a reinforced burlap/hessian webbing eyeleted at the head with ties, & a tail pocket to accommodate the chain weight, as mentioned by Dave.