With the salvedged edge meshs you will be wanting it to be at least 14 mesh deep(wide) the factory net is 16 mesh deep. As for lenght of net that all depends on how long you want to keep knitting.
25yard long = 900 rows of 14mesh deep =12,600 meshs thrown
50yard long= 1800 rows of 14mesh deep = 25,200 meshs thrown
74yard long= 2700 rows of 14mesh deep= 37,800 meshs thrown
100yard long=3600 rows of 14mesh deep = 50,400 meshs thrown
that is how much work is involved in making your own net, still want to have a go at it?
As chalkwarren said, a hand made net is the dogs do-dar to use