A 3500 series plano is just about the same size as a traditional large fly box .
I use a mix of both in my kayak bag as they switch out interchangeably. You can put ALLOT of flies /bugs/poppers in a plano 3500. Just be sure to sort them by type, size, etc. so you keep them organized. That also makes it easy to swap out different box's based on the days plan, location, season etc.

FWIW, I use a 3500 series tackle bag to hold all 4-5 box's and gear ( in the pockets). Keeps things well organized and versitile. Works well for my warm and saltwater fishing needs anyway.