I think the physical dimensions of the console are less important for space considerations than whether or not the drive is side or top loading and the console needs room to circulate and exhaust air.
While the physical size of the PS3 console and 360 power supply are generally exaggerated, I don't think there is much dispute that these consoles waste an additional amount of space at least equal to their size in clearance around and behind the console. And while the Wii may be efficient, it runs notoriously hot in its WiiConnect24 mode, which is suspicious given that it doesn't really *do* anything -- at least I haven't seen the blue glow from the slot loader on my machine in at least a month.
Cartridge based units suffer from the top-loader, which requires clearance not just for the height of the cart, but to get your hand in there too. Clamshell CD drives suffer the same problem.
Lastly, the design style will dictate whether you can place one console on top of another, which is a huge benefit if you own several consoles and want many hooked up at once. Unfortunately, most consoles are designed with curved tops or and absence of flat surfaces to discourage this, either to prevent the owner from stacking consoles that generate a lot of heat, or to discourage customers from setting drinks on top of the hardware.
Considering all of these factors, I think the only two consoles that were designed really well for the small entertainment center were the original front-loading NES and the Panasonic model FZ-1 3DO. The original PS2 I'm throwing out on the basis that it was ribbed for no ones pleasure coupled with the fact that its drive whirred louder than a jet engine no matter how new or old it was. :p