How large is the largest paver?

That pattern should be ok, think about doing a stretcher course border (head to toe) of a different color to frame the space.

The largest paver is 12x12. The contractor wants to use a 4x8 paver for the border, length ways, ie the paver extends 8 inches in. Bit concerned about that. Actually, that's probably what they've done in the photo above. Only leaves about 7 feet width on the narrower side.

I think you should be fine. 12X12 should give you several full rows across. I like the pattern and the stone color. Do you have the capability of drawing it out on graph paper to see how it looks to your eye?

Now I'm a bit worried.

The contractor has left some stacks of the pavers nearby in prep for paving tomorrow...

I was under the impression they'd be lighter. Here's a photo of a palette of them I took at the supply store...

The ones at the store look lighter to me. I asked the contractor to use the lightest ones he has, but I'm not sure how that will turn out. I really wanted something light, lighter than the concrete. Concerned the pavers look a LOT like the concrete.

There are a lot of variations in the colors. I think they'll be fine. Did he say he picked the lightest available?