My opinion, lets call it a minimal guide so to speak.
What I suggest:
A tarp length minimal 8" at each end (so 16" total) longer than whatever my hammock length, when hung, is. So a hammock that is 8' long when hanging from the trees, means (at least to me) a tarp with a ridgeline of 9'4".
Tarp sides: Enough to cover your hammock AND insulation in a driving wind. So if your hammock sag, from ridgeline to the lowest point in your hammock WITH YOU IN IT. So a 4' sag & the above ridgeline means you want a 9' 4" X 8' tarp, or (an easier way to do it) a 10' X 8' tarp.
IMHO, this will get you through a pretty strong storm if you cant find a protected place to hang.
This will not protect you from a storm where the rain is "falling" horizontally, or coming down hard enough that it "Bounces up", because remember that as you spread the bottom of your tarp, it gets further from the ground, so less side protection.
If you have a hammock like a Grand Trunk UL & you like a flatter sag, you may be able to get away with a 8' X 6' tarp, and be able to stand a "storm of the century".
What I did: bought a "blue tarp" about the size I thought I would like, I think I spend about $9.00 for it. I got a 12 X 10, & it seemed a little short along the ridge, but when I measured, it fit the above criteria with inches to spare, so I got (well, ordered) a 12.5' X 10' tarp, what came was a foot longer than I had planned. I LOVE IT!! When I wear it out, I'm getting the same size!!
With that tarp I can close both ends, & at my height of 5'6" can almost walk upright with it staked TO THE GROUND as low as the tarp will go.
Tarp weight including stuff sack is 2 Lbs.
I joke that "I can house a family of 4 & they won't have to see each other for a week".
So for a solo hiker, it's a great place to hang out during a bad storm, cause I can actually walk around, stretch, cook (done it ONE
time) & stuff. When hung the way I usually do, I can stand up straight & not touch the tarp.