I have both a 6x8' and a 10x12' silnylon tarp in my BOB. The 6x8 is for making a rain shelter while pausing from moving and the 10x12' is my shelter building tarp. it's big enough to build a shelter with a side or two if needed for weather protection of a sleeping/cooking area and big enough to be comfortable for several people if stuck in one spot for several days,
Back in my SAR days, I carried a 10x10 blue tarp on the out side of my pack. It was often used as a overnight personal shelter tarp for me and maybe a team mate (if needed) and as a team shelter if we had to hunker down or as a shelter while treating or packaging a patient.
I was picked on by team members until I used it as a 3 sided A-frame shelter one very cold and rainy 2 night training session while others used large trash bags. Those in the trash bag shelters were often wet and cold, while inside my tarp I was warm in my hand warmer heated 2 pound sleeping bag and dry. The 3 sided A-frame design also acted as a ground cloth and it was long enough for me to pull the ends in and basically close them off. During the day I opened the trap up and hung it on para-cord from trees as a team weather shelter.
Several others added 10X10' blue or the more expensive nylon tarps to their packs after that training.
As far as I am concerned, a large tarp in a requirement in a BOB bag.