Nearly 20 years ago I had a friend sew the tarp I have used the most for backpacking, an 8x10 flat sheet of kite cloth, also called spinnaker cloth. It was the lightest weight till sil-nylon came out. Never weighed it but am guessing under 8 oz. total and rolls to the size of a paper towel core.
I would make it at least 9x12 if doing it again.
For a solo bivvy I have considered carrying one uncut and unsewn piece of fabric. 60 inch wide would be minimum, 8 or nine feet long. If you could find 72 inch wide fabric it would be better. Melt the cut edges to stop fraying and either sew in some ties or put in some lightweight grommets. My Golite poncho serves that role for me now.
Do you want a minimalist tarp or a larger one with more coverage and creature comfort?
I have designed a flat tarp with ties to convert it from flat to enclosed to the ground all around. Never sewed it and am less likely to now. I anticipate that Seek Outside will offer something similar one of these days,
a natural progression.