Yes, that can really fit four full-length nVidia Titan (or similar) GFX cards.
What the hell is the obsession with size? I'd like to see a concept that shows the modular Mac Pro as big, or bigger, than the old cheese grater tower.
Edit: Yes, I see it says "two slots for full-sized graphics cards" - with no room for anything else inside. Not that it could run two full-sized graphics cards considering there is no room for cooling or fans.
No kidding. There's no reason for the Mac Pro to be small. It could be twice the size of the cheese grater, that would be fine, if it gave optimum performance. Low noise is nice, and the 2013 model did a great job of that, but even that is probably not the highest-priority slider on the set of things to trade-off. It needs to be the fastest machine you can make with the highest-performance Xeon available at the time, with the fastest drives and memory. And....although I don't care about it (software developer), it certainly needs to be able to host a high-performance GPU, and be upgradeable. Ideally that would come about via off-the-shelf graphics cards, but....that might be asking too much of Apple. As long as there is an upgrade path that's competitive with the top-shelf graphics hardware out there.