I think you want size 2 in the "3,2,1" option. You definitely don't want 2/0, 3/0 etc...as they all look enormous.
For this wallet I used just over 6 stitches per inch and I used bonded nylon 207 machine sewing thread (T210). Size 207 thread is .508mm in diameter to give you a reference point.
I hope that helps!
Joe