For a piece that large, I would consider a 1 1/2" or even 2" bar. This is the shoulder height, which is what you will gallery wrap around. Giving you 1 1/2" or 2" depending on the frame of edge.
You can consider mounting the hanger two ways.
1. traditional method of mounting the hangers to the edges of the frame, just like a framed print. This will force the piece to hang with a slight down angle
2. Use eye screws and mount your hanger inside the frame. This allows the piece to hang flush to the wall and makes for a nice look. I will state that during the
hanging with this method, it's very easy to damage the canvas due to pressing on the inside and creating a bump or bulge that will be very hard to get out.
Also for a piece of this size, you might want to consider corner stiffeners and at least one inside strengthening brace in the middle of the frame.
Paul Caldwell