a lot depends on the type of leather and style of bag but a good wt is 5oz most bags can be made with a min.of 4 sqft also 5oz cow is much stiffer than 5oz elk
A lot of guys use 3-4 oz. I prefer 4-5 oz myself.
Yep, about 4-5 square feet should be plenty. Don't forget some of that is strap and needs to be long.