First of all, shards build always delegate all it unknown options into Crystal.
Then, if not add --no-debug, it always include debug info, add this option will reduce file size significantly.
Add -Dpreview_mt only if you know what you want.
--static not always work for current version Compiler, in fact, AFAIK, it only work when build static binary from alpine docker container, for build a linux static binary or an universal binary for Darwin without use docker, you probably want to check Use zig cc as a alternative linker (draft version) - #2 by zw963
you should always use --release when release a binary.
For --production, as said by shards help,
--production same as `--frozen --without-development`