I've been planning to build a new server, and have acquired a suitable motherboard with CPU, and HDD backplanes for 10 drives, what I need is RAM (no problem there), drives (ditto), power supply (same), and a case that preferably fits the contents but is otherwise as small as possible.

So, the Goglodyte hivemind wouldn't be able to help me find as small a case as possible that fits a Mini-ITX motherboard and six 5.25" drives (the two HDD backplanes uses three slots each, and I have no need for an optical drive). The smallest I've been able to find are miditowers at about 50cm height and depth.

When I am looking for specific (read : weird) cases, I usually check on Lian Li first.

So far, for mini-itx, I just saw 2 x 5.25 slots. (edit : it seems in the mini itx range it's too much cramped)

Just off hand i'd say you want a full tower. A full tower would give you like 6 slots on top which you could put hard drives or optical media...

Course there's specialized cases just for certain types of internals if you wanted a server farm or something...

Lian Li PC-Q35 might be worth a look, not quite enough 5.25" but maybe you can make it work? see if any of their array of accessories will make it exactly what you need...

http://www.lian-li.com/en/dt_portfolio/pc-q35/

http://www.lian-li.com/en/products-chassis-accessories/#all/1/list

EDIT: nvm, quoted dimension is a mistake, was given without front panel

Hmm, I did see that Lian-Li chassis, and from what I can see, a sixth 5.25" slot could fit below the other ones. That might be workable with a dremel, but not having worked on cases that way before, I should probably train myself on ones I'm about to trash first.

To more accurately show what I have so far:
Icy Box IB-525SSK. Two of these boys, each fitting 5 3.5" drives in 3 5.25" slots for a total of 10 drives in 6 slots. I won't have more drives than fit in these, so 3.5" slots are rather useless.
ASRock C2750D4I. Due to features on the board, installation and even BIOS setup will be handled over the network.

I could really get any cheap miditower case with Mini-ITX support, but I get annoyed that they more often than not have no less than 9 5.25" slots, making the case quite a bit larger than what I need. Another option is, of course, a 4U rack case, that's not only bigger than I prefer but also less cheap, still an option though.

How about the Bitfenix Prodigy?

Only a single 5.25" slot.

The backplanes are to hot-swap the HDDs whenever necessary? so would the PC-Q35 and similar cases be too small, simply because there wouldn't be space to easily pull the HDDs, they would collide with the PSU or CPU heatsink?

This is bigger than you want in terms of litres capacity, but at least the footprint is smaller: http://www.lian-li.com/en/dt_portfolio/pc-d600/

It's also extremely expensive, but if the layout looks useful maybe there will be a less expensive plastic copy somewhere?

The HDDs are pulled out and inserted from the front, so once the backplanes are mounted, I won't have to open the case. They're about as deep as an optical drive, just three times as high, so if DVD drives fit in the case, these will too.

Yeah, we have a double-wide case for a retired server at work, I thought of taking that at first but decided a common miditower would suit me better.