Not a certified medievalist, but play around in 1300s England. I've mostly seen ells used for the finished cloth measure, but could you cheat and say 'enough thread for howevermany ells of cloth'?
For whatever help it may be, 'cloth' in England meant woolen cloth. Linen included flax linen, hempcloth (also called canvas, cognate of cannabis), and nettlecloth.
Thread comes not on spools but in clews / clues, rolled bundles of thread. That's (here's your useless knowledge for today) the thread Theseus followed out of the labyrinth, and why we use clues to solve a mystery.
A friend of mine, when she learned to spin, gave her friends sample threads rolled into little bundles, saying 'Here, have a clue!'
-Barbara