I’m about to attempt to make my first tarp. It’d be for me and my girlfriend to share. We’re planning on accompanying the tarp with breathable top bivies. Neither of us have any experience with tarps. I made a quick mock up with cheap fabric using the 5 Yards to SUL plans in order to size out how big we’d need to make it. I’m assuming we’re correct in thinking the roughly 8.5’ x 6.5’ (beaked) tarp will be too small for the two of us. How much more do we want to add? I see that GG SpinTwin is roughly 9.75’ x 5.5’ with no beak. Should I just copy their specks to be safe? Any idea how much silnylon I’m going to need?

I'm certainly no expert in making tarps, and wouldn't try even if I knew how to thread a sewing machine. But I do know that any question that includes "new to tarps" and, "girlfriend" must be answered with the perspective of 80% ultralight philosophy and 20 percent Dear Abby. Don't go small.
My wife -- who I still think of as my girlfriend -- and I use a 10x12 and love the space it provides, especially when it is raining and cold here in Southeast Alaska. I've been hinting at the purchase of an Oware 10x10 for our shorter and "drier" trips. She will come along, as she always has, once I can prove that we and the backpacks, or drybags if kayaking, will fit underneath.

Welcome to the MYOG zone, but be aware that it is a slippery slope, he-he.

Ditto what Joe Reeves said, plus keep in mind that since you made the tarp, you know that you'll be capable of reducing its size when/if you both agree that is needed.

Regarding how much silnylon you'll need ... width of the fabric varies but there are many sources that are 62"-65" wide. Anything wider than 5 feet will require two pieces and a ridgeline seam. Fabric is almost always sold by the linear yard (at whatever width it is), not square yard, so a 10x12 would need 24ft = 8 yards. 10x10, 6 2/3 yards.

Enjoy your project!

Like Jim said, the roll widths will very somewhat. Since you will have to sew two pieces together, it makes the most sense to do it down the center to form a strong ridgeline. Not wanting to waste material, I found it difficult to bring myself to cut it to make the tarp narrower (say 8 x 10). Because of that, I found it easier to decide which length I thought I would want and then utilize the full width of the material on both halfs, be it 60" to 65" per side. I think I ended up with a tarp 10' x 10'. As noted in one of the previous posts, you could always trim it down. For 2 people the extra width comes in handy and you can offset the weight penalty of the extra width by staking the corners and side guyouts directly to the ground, eliminating the need for guy lines to the ridgeline ends only. Consider ordering an extra 3 yards of material and make as easy solo tarp about 5 x 9 (very light). It goes faster because there in no ridgeline to sew.