The size I picked isn't tablet size, it's 3.5" drive sized. That's a bit bigger than a tablet. Though, I'd go as far as half-height 5" disk drive sized.

As for the 1.8" drive socket, I don't intend that to be for a HDD. I intend that to be for an SDD, but there are mobile devices out there that use 1.8" HDDs. They're QUITE popular, actually (standard iPods).

I don't see the need for a hardwired ethernet port. I see the need for rock solid wifi, maybe two interfaces (depending on what it takes to definitely have a wifi access point, and optionally also have a wifi client, in the same box -- I don't know how many wifi devices can do both at the same time). Ethernet can come from an USB ethernet dongle, as far as I'm concerned :-} (if you're talking about using it for management and configuration, then I'd say "make sure it has a USB client interface" and do your management through that)