Ok, having stopped by grumpy, I found the following in the 190.36 read me (please could we add the 'Implementation Limits' section to the API reference?).
The maximum supported resolution is 4096x4096.
The following surface formats and get-/put-bits combinations are supported:
VDP_CHROMA_TYPE_420 (Supported get-/put-bits formats are VDP_YCBCR_FORMAT_NV12, VDP_YCBCR_FORMAT_YV12)
VDP_CHROMA_TYPE_422 (Supported get-/put-bits formats are VDP_YCBCR_FORMAT_UYVY, VDP_YCBCR_FORMAT_YUYV)
In all cases, VdpDecoder objects solely support 8-bit 4:2:0 streams, and only support writing to VDP_CHROMA_TYPE_420 surfaces.
I guess that covers it, thanks.
Sometimes it's tough on a list when people try to help but don't quote where the info comes from - sometimes a definitive answer is what people need, and the developers/company reps on this list don't have time to provide specific answers to everyone. I'm going to have to try to be aware of this myself. I got quite the catty smack-down on the decklink list a while back...