Yes, get the 40 denier tricot nylon. I've only ever seen it in 108"-110" widths. As far as length, always go longer, so at minimum get enough fabric for 14ft. I have multiple silks for shorter lengths I never use any more and wish I'd gotten length for my full rig height, which is 20-ish feet.
For length, take your ceiling height and multiply by 2 and then divide by 3 for yards. Add another yard or two. So for 14ft, it would be 14*2=28/3=9.33 yards. Then depending on whether your fabric is for hammock or silks (It wouldn't matter to me, I'd add the same either way), add another yard or so. At min, I would buy 10.5 yards. Probably more like 11 yards.
As far as the fear of buying fabric from a fabric store, I cannot fathom the difference. Assuming you are getting 40 denier tricot nylon. I don't care if someone has had their fabric tested for strength, you would have to batch test every bolt of fabric to be able to claim strength, the same way every batch of concrete is tested.
I do full aerial with drops and am perfectly comfortable with Fabric.com fabric. I am more concerned with the climbing rigging because it is not meant to be "fallen" on continuously. Because Fabric tears, when it fails, it doesn't fail catastrophically. Steel deforms and aluminum snaps. Even though the aluminum is rated way higher than the fabric is for strength, it's the piece that make me the most nervous.
However, if you aren't doing drops, my concern would be pretty low to non-existent.