+1 on the 8x10
Not much extra weight. Lots of extra protection and useful pitch flexibility . . . of course, if you're both <6' tall and <150 lbs and serious cuddlers and don't mind getting wet and cold sometimes . . . what the heck? Actually, a 6x10 is still long enough to make a pretty good pyramid, and wide enough, that if you rigged some kind of an extra sheet/curtain to cover the opening to reduce blown in rain, you might get along pretty well in your 4x7 sleeping space . . . assuming you don't get into one of the long rainy weeks from hell that are so common in the northwest. I'd sure as heck do lots of experimenting with pitching, and working inside the pitch, before I committed to a tarp that small for two.
Hmm, two (or less) extra ounces split between two people to double the usable under-tarp area . . .
You could always pick up a bigger tarp for more protection later in the fall as the weather turns in the NW if you are going south to north. Then you wouldn't have to carry the extra ounce the whole 2650 mi.