I use crochet thread, pearl cotton, embroidery floss, or Pretty Punch yarn.....
Have tied up to king size. but usually use blankets instead of batting for tied quilts, as you can space much further apart--plus NEVER any bunching, etc.
My favorite tip---a drop of fray check on each knot to keep them tied----drop it on when trimming them so I know it is done! Wash them and wash them, and they stay tied!