How about cutting the blanket using an old carpenter's trick (question)? Let's say the man is 6 ft tall, and wants at least a 6 ft blanket to give him some head room and comfortable feet. However, his blanket measures 5 ft x 3 3/5 ft (a reasonable size, and the reason for this tricky number will become apparent). Now viewing the blanket from the sky, and with the blanket oriented so the top runs 3 3/5 ft across and 5 ft down, he could cut the blanket as follows:
1) Make a vertical cut at the top of the blanket one-third of the way in from left, stopping halfway down the blanket (a vertical 2 1/2 ft cut).
2) Continuing from the first cut, make a horizontal cut one-third of the way across the blanket (a 1 1/5 ft cut).
3) Continuing from the second cut, make a vertical cut the rest of the way down the blanket (a 2 1/2 ft cut).
Now, rearrange the left most piece just cut but moving it down and to the right so it joins up with the other piece. Sew them together, and now you have a blanket 7 1/2 ft long and 2 2/5 ft wide, plenty of length to cover the feet and wide enough to cover the guy provided he sleeps on his back. The reason I chose those tricky numbers is to get the proper area. The area of the cut piece is (1 1/5 ft) x (2 1/2 ft) + (2 2/5 ft) x (2 1/2 ft) = 9 ft2 = 1 yard. If you tilt the blanket 90 degress, this yard could be said to come off the top.