I have pieced fleece to back a king sized quilt, why would you think it can not be pieced? I used a 1/2" seam and opened it to keep the bulk down. I love fleece backings ( actually I love flannel, but my family loves fleece) so use fleece a lot... Never thought about ( or experienced) any problems, piecing together to make the right size. My mom makes fleece socks every year for family members gifts and passes on the leftovers to me so sometimes a fleece backing will be very pieced. I sew pieces together until I have the size I need.