WOW you're in quite the predicament!
We rotate between NV Instinct, Wellness CORE, and Taste of the Wild. The Wellness CORE kibble is HUGE. I'm pretty sure it's one of the largest foods I've seen and I've heard many people complain about the size. TOTW kibble is smaller. I would say it's about a little bit bigger than Instinct Chicken but it's flatter. I don't know if that would work or not? We have tried all 3 flavors of TOTW and they love all of them.
It seems like anything I would suggest has already been tried.
Good luck on your search!