On the DC side of things, I think the second and third Flash Omnibuses are about perfect size. They're big enough to be worthy of the title omnibus, but still easy to handle. I'm really glad we didn't get all three Flash Omnibuses in one huge $200+ behemoth Omnibus. That would have probably been way too large, unless they were printed on ultra-thin paper, and no one wants that.
Also, event books like Infinite Crisis work really well as a larger omnibus. If it were put into an Infinite Crisis omnibus and Companion omnibus (like Marvel does sometimes) it could work, but the reading order probably wouldn't be as fluid. That's something that I really like about the Infinite Crisis Omnibus, or I guess I will when I get around to reading it.