There are two command-line options to control image size for lossy compression. You can use -side to specify the maximum number of pixels on a side. Similarly, you can use -count to specify the maximum number of pixels total. Both of these options imply using lossy compression.
./Adobe DNG Converter -side 2000 MyRawFile.NEF
produces a lossy compressed DNG that is limited to 2000 pixels on the long side.