The 'Flush files' option you'll find in the DCC/Options/ menu will make mIRC to write incoming files (more often) to disk, like every 500k. This makes sure you have at least a part of the file when the download gets interrupted.

When you DCC Send a file by hand from the DCC/Send/ menu you can set the minimize option. That option will be saved and used for all sends, including sends from the command line, scripts or DCC Server.