I would probably do a 1/2" or 3/4" seam. Are you wanting to avoid putting a binding on it? I just did my first tied quilt but I didn't birth it (which is what you are wanting to do on yours). I left my backing a little bigger than the top and pin basted then tied. I then bound it like all the other quilts I have made. I was afraid to birth it because I wouldn't know what to do if there was excess in the backing when I went to tie it.