I'm eagerly watching your build.
My wife and I are planning to build a similar pool for our new home. Rectangle with spa inside. Our initial thought was 14 x 35 for the pool with a 7 foot spa. We had a 14 x 28 pool at our previous home with no spa and thought it was a very nice size, thus add 7 foot of spa and ledge and you get 14 x 35. Our water depth on the 14 x 28 was 3.5 -> 6, and we were happy with that too. Our initial bids have been too expensive, $130k-$160k, (including decking, auto-cover, and nothing in concrete happens in TN for under $75k, etc...), so we know we're going to have to scale things back a bit. Maybe 14 x 30 with a 6 foot spa will work for us and help trim costs a bit.
Why did you choose a 6 foot size for the spa? We don't really know what we want in terms of size for that and the ledge.
If it helps, I noticed in your previous post that you get a tiger shark with your build. We had one of those and I really liked it. It did a great job in my 14 x 28 rectangle. It was a screened pool, so no big debris, but dirt and worms and all that stuff it did a great job. I did get a caddy for it, you may want to consider this, the tiger shark doesn't seem heavy unless you have to carry it somewhere when its wet, dripping, and you're old and weak like me. Easy to order one later if needed.
Good luck with the build, can't wait to see it finished. Do you have kids who will be using the pool too? I ask because the 14 x 28 was great for my kids who are small for age, and both under 7, but I did wonder if the pool would seem smaller once they were bigger kids, then we moved before we could find out.