Problem with a big size render (Cycles)

I need to do a big size render for a booth. When doing so in full HD (1920x1080) the render takes a long time, but finally gets done. If setting 4200px height (1,5m approx) the render is taking more time, and the final result is a plain color (environment color).

Trying it with a simple model (gold sphere) turns out to be the same result (environment color).

These are the technical specifications:
i7 3.60 Gh
32 Gb RAM
GeForce GTX 980 Ti (x2)

Any advice to solve it?
Thanks in advanced.

The problem is related to the resolution. Many resolutions work, some don’t. You could try some different sizes in the region of what you need.

This is on my list: