There is a restriction for Maya 2016 that doesn't allow Maya to use more than 4k textures. As a result, the size of Viewport 2 MRenderTarget is limited to 4096x4096.
We've opened a back door to remove this restriction and allow users to use the maximum texture size supported by their graphics card. This is now possible due to a new environment value introduced in Maya 2016 SP6 and 2016 Extension 2 + SP1. This variable is named: MAYA_VP2_USE_GPU_MAX_TARGET_SIZE, you can set its value to 1. After setting it in your environment, Maya will use the max texture size of your graphic card instead of 4096x4096.
You can find the max texture size in Maya output window:
In Maya 2017, we've removed this restriction and the environment variable is no longer required to unleash the full potential of your graphic card. Enjoy:)