Apple has made the new Mervericks desktop background image available to the public, and it’s absolutely huge. At 5120 x 2880, the image is the same size as that suggested for a Retina display 27-inch iMac, or a Retina Thunderbolt Display.
While some may doubt Apple would reveal a screen size before a product’s launch, there is precedent; with the second developer preview of OS X Lion in March 2011, Apple included several large, Retina-sized wallpapers. This was long before it publicly announced its Retina display MacBooks, which were officially launched at WWDC over a year later, in June 2012.
Apple did announce on Monday that the new Mac Pro would support 4K outputs to external displays, so these wallpapers may simply be preparation for that. However, the next logical step for Apple is to bring its Retina technology to larger Mac displays.