I just bought a Kifaru Sheep tarp for sun and rain protection for hunting and glassing. It is a 5 x10 tarp and weighs 9 ounces. I don't bivy hunt and it is for solo use, so its size and weight works for what I am using it for. I always try to keep weight to a functional minimum, mostly because I am getting older and less is more.
For glassing and rain protection I would go smaller. If you are going to be sleeping under it, I would probably go for the larger size tarp, which I think weighs around 25 ounces. The bigger tarp weighs about 5 or 6 ounces more than the 7x10 but would probably be worth it for a sleeping shelter.
As an alternative, if you need a 10x10 tarp, I would also consider spending a bit more and getting a Seek Outside DST tarp. It is a 10x10 tarp, weighs 18oz and will take up a lot less room in your pack. Extremely functional tarp.