Hardware acceleration is limited to the following frame sizes:
1920x1152 (for max width resolution)
1856x1200 (for max height resolution)
Larger resolutions will fall back to the software implementation. Encoder and decoder initialization will return MFX_WRN_PARTIAL_ACCELERATION if the hardware cannot support the frame size.
Related threads which may be of interest:
Aspect ratio limitations are described along with mfxFrameInfo AspectRatioW/AspectRatioH in the manual.
Please let us know if you have any more questions.