Nokia RM-877 just got approved by the FCC and judging from its LTE bands, there is no doubt this is a At&t bound device.
There isn't much to go on as Nokia asked that most of the specs documentations remain private... for now.
Just this image is all we have
The image shows a device that is 130.35mm long and 71.4mm wide, I cannot verify the thickness at this time but when stacked with earlier Lumia models, the Lumia 925 is 129mm long and 70.6mm wide, while the Lumia 920 is 130.3mm long and 70.8mm wide.
So it looks like this RM-877 device, while it is almost the same length as it Lumia 9xx counterparts, it will be wider, meaning it will have a slightly bigger screen, I'm thinking 4.7-4.9inch, unless Nokia reduces the amount of space consumed by the borders and the capacitive screen, then we could be looking at a 5inch display.
Images have been surfacing of a Lumia device with 41MP camera, and Nokia has already sent invites to the Press for the July 11th event where they will unveil this device.
The above was yet another leak showing a mystery Nokia phone in production.