stuff sacks

Jay's MYOG series includes a 4×8 stuff sack, so if nothing else you could extrapolate for larger sizes. Maybe you've already seen this.
http://www.backpackinglight.com/cgi-bin/backpackinglight/make_your_own_gear_5_yards_to_sul_part_2.html

This fomula's is lifted from Thru-Hiker

[(desired diameter) x 3.14] +.5 = fabric width (desired depth) / .75 = fabric length or Length = ((desired depth + 2) + width/2)

Check out his site as he sells them in kits. This is nice because you get everything you need and the price is the same.

JFF

I like my stuff sacks to have a round bottom, so I cut a circle 1" bigger than what I need for a bottom, to determine the length of your fabric for the body just multiply your bottom size by pie (3.14) be sure to allow for seams and anthing else (such as grommets)

If you get some cheap fabric, you can quickly make one to experiement, then cut around the pattern on the expensive fabric. I use a very simple tacking stitch for the model, which is quick to make.

I buy old bedsheets from charity (thrift) shops. At something like £2 a sheet, they are excellent value.

My method is as follows:

I multiply the desired diameter of the stuff sack by 3 (techically pi, but it's a stuff sack after all) and add 1 inch for seam allowance. So, a 4" diameter stuff sack would require 4×3+1 = 13 inch wide material.
I then cut the material the length of the finished stuff sack plus the diameter plus 1 inch for seams. So, 10" stuff sack would be cut 10+4+1 = 15 inches.
Next, I fold the material in half so that it measures 15"x6.5" in my example above. I cut one end of the doulbed fabric in a semi-circle shape. This will be the bottom of the stuff sack.
I then unfold the piece and sew the draw-cord in along the 13" side by folding over about 1" of fabric.
Finally, I double the fabric again as previously described and sew the one long seam that goes around the semi-circle bottom and up the side to the draw-cord end.

As a bonus to tackling DIY projects, one usually ends up with about 3 new stuff sacks!