No. I have a window box on the front of my house under the upstairs windows but there is nothing on the ones below nor on the other ones on that floor. I don't think it looks "odd".

I don't think you need a window box under every window. In fact, sometimes less is more -- makes the ones you have more special.

I think they'd look good there and would complete and ground the look of that space.

I'd probably do them the same size as trim.

You mention that you'd make them big enough to fit in a plastic pot. Not sure how you are planning to do yours, but years ago I started putting plants in individual pots in my window boxes. Then if one died, I could just pop it out and throw in another. They also seem to be healthier than when I did them all in one big planter. Much lighter as well. No matter how you plant them, mulch them well as they dry out quickly. That is all my unsolicited advice for today :)