A quick note on supplies:
A good fabric store should have a reference on hand regarding amounts of fabric necessary. Bring in measurements and/or a photo of your furniture, and if there's anyone helpful working there, there shouldn't be much of a problem figuring it out. Mine, a smallish loveseat, required only 10 yards. Prints and geometric patterns generally take more, because you'll need to match the pattern on all the pieces.
As far as thread goes, go with something pretty heavy-duty (like a polyester) for the top, and then a lighter-weight thread to go through the bobbin, so it doesn't get jammed. It looks like I will be going through about 100 yards of thread total for this project (that is, 100 yards for the top and 100 yards for the bobbin). I could simplify and say 10 yards of thread for every yard of fabric you will be using, but I don't know if that's just a happy coincidence with my sofa or not.