Direct3D 11 DXVA decoding documentation lacks accuracy. The API, sadly, lacks other things too but it is a different story.
D3D11_VIDEO_DECODER_BUFFER_DESC as defined in Windows SDK:
MSDN documentation lost
DataSize field, which is – ironically – the most important one along with buffer type.
D3D11_1DDI_VIDEO_DECODERR_BUFFER_DESC structure has both fields but the Members section has an obvious copy/paste typo:
The structure and the API itself is presumably not so popular.