You can create a HBITMAP with CreateDIBSection and copy the bits from the final
D3D11_MAPPED_SUBRESOURCE.pData (from the end as bits are inverted (from image size - RowPitch))
Then you can resize the HBITMAP with StretchBlt
For example, I did a test by dividing the size by 3 from the screencopy with StretchBlt on the main window DC :