On the extremely small end, I was looking at the Lone Industries L2
. Of a more reasonable size, I was thinking of the Silverstone SG06, SG08, Lian Li PC-TU100, or others. I'm open to suggestions here, as I haven't really picked out a case.
If the Lone box is your current favourite, what about the Ncase M1?
Anyway, about the TU100, I just think it has below average thermals, so I'd prefer it with an IGP setup, rather than with any discrete graphics.
The two Silverstones look like, on the contrary, way bigger, if you may like horizontal enclosures the ML05/06 are more similarly sized to the Lian Li/Lone ones: about Silverstones, I would advice the freshly reviewed Raven Z/Milo 7, but all in all it's just a slim mATX tower (they have similar height and depth of enclosures like the Antec NSK-3480 or the Silverstone SG03/04).
As a suggestion, give a look to the Jonsbo enclosures: in U.S.A. they are sold under the Rosewill brand (Newegg, I suppose).
I would think also to the EVGA Hadron Air: it's not as tiny as a TU100, therefore it sports less limitations (but you can't use Picobox and the like with it).