I guess I'd make that call based on how reliable the hardware is, but generally I try to go for just one box no matter the size. Just because it's an easier setup, easier planning, documentation etc. And usually less money. But there's really nothing wrong with doing your setup.
If your main concern is uptime, I'd put one box as central router with multi-WAN and put the other one as HA to automatically take over if the first one fails. I would make a LAN network (VLAN1) for devices such as switches, AP's etc, then two or more VLAN's for users. In the past when I've built large networks I have sometimes created a 22-network (255.255.252.0 subnet mask) just to get a few extra IP's, and sometimes I've limited them to about 50 devices per network, depending on the type of traffic. Smartphones and such is good to keep down in numbers as they broadcast a lot of traffic, but if there's *nix devices it doesn't matter as much.
The main thing I go for is to try and keep as much as possible with software, since it's easier to replace one box and restore config than to troubleshoot and replace several boxes. Correctly done, you can even replace a router on remote with a novice customer moving a cable or two.
