I often use sheets for backing. The smaller the count the better. Wash, Wash, Wash first. The one thing I discovered is that the sheets are not always "bias" ready.
I rip by tearing all of the edges first to get the right bias. Press out the sides. Sometimes also need to finite edges with a rotary cutter. Have fun. I made a lot of large lap quilts, so usually buy the twin size, $5.00 at Wally World. :D