I'm no expert in this area yet but here'a a bit of background info:
There are 3 major form factors for cases:
ATX - Stardand 'big' size. Cases sizes range from normal to huge.
Micro ATX - These are quite common and typically cut a few of extras of the normal ATX boards, but don't typically loose any common functionality... so you can still build a full PC around them and also keep the size to a respectable level. Typically they look like a nice compact regular case, often used by OEM builders like DELL. The Mini P180 to linked too is a Micro ATX.
Mini ITX - These are now getting quite small. You can still build a full machine around them but you will start to pay a price premium for the size on the components needed to make it work. A good example of Mini ITX cases are the
Silverstone SG05 and SG06 more can be viewed
here Mini ITX machines don't always support a graphics card (relying on on-board graphics) or only support low power/cards with physically small dimensions.
If building a gaming PC around a Mini ITX set-up you have to be very careful you make sure everything will fit (check all dimensions), make sure the power supply will be able to power all the components (everything ideally want to be energy efficient as the PSUs for these cases are normally lower wattage). You also have to consider the cooling the case cna provide for putting high power components into a very small area.
Finally...
Nano ITX - These are ultra ultra small (about the size of 3 Jewel CD cases stacked on top of one another. They're fairly new and can provide enough power for HD playback at present, and are normally very low power. Good for low power home theatre PCs, but you can't really game on these.