You can go two ways, either use HDDs in RAID config, in which case any ATX mobo will do, most have 4-6 SATA ports.
If you want to go the mini-itx route, you can use a NAS with a Gigabit ethernet card. This means you can have a really small unit.
Interesting - thanks for the reply.
I think I want to keep the RAID storage in the same case - also because NAS software can be finicky and reliant on the speed of the NAS CPU (I've encountered issues before).
Would there be any reason not to get an mATX motherboard and case that supports that size? Is it unnecessarily limiting?
It would be nice if the thing could fit in a media cabinet slot, just like a receiver. But not necessary.
You'll want to go with a HTPC case that accepts a full-ATX motherboard. These normally have a sufficient number of internal hard drive bays and still allowing for decent air flow. Most of these cases accept either full-ATX or Micro-ATX motherboards, so that would be a matter of which board offers all the features you want.