8.5×8.5 -or- 8.5×10 Flat Tarp for Two?

Definitely go with the 8.5×10. The extra couple feet makes a huge difference for keeping wind blown rain out. But I don't use a bivy so your usage may be different.

10 ft for sure. Chances are you'll end up using one pitch you like and sticking with it. Go catenary if you like a-frame.

"10 ft for sure. Chances are you'll end up using one pitch you like and sticking with it. Go catenary if you like a-frame."

That's why I'm debating between the square and rectangle. I'm afraid with a rectangular tarp, I'll only ever pitch it in the half pyramid. I've owned both previously, and used them for solo, and found the square tarp to be way easier to pitch in different pitches (flying diamond, half pyramid, a frame). I'm just wondering if the square would be enough coverage. And it isn't trending well, so far.

I vote flat square, but use an umbrella for extra head or foot protection.

I have a flat rectangular cuben tarp. I've used it on a couple trips with two people and really appreciate the extra room–i.e. not much extra room, but less room would be very tight.
What trouble have you had pitching a rectangular tarp in different ways? I've found it easy to set up a flying diamond, half pyramid, and a-frame–with the flying diamond being the easiest (albeit assymetrical).

I prefer an A frame pitch so that's a pretty easy choice for me.

You could have two extra tie-outs added (four total each side) 1.5' from one side of the 8.5×10 and end up with a tarp that does it all.