$ret.err, $ret.ownerid, $ret.owner, $ret.groupid, $ret.group, $ret.size, etc.

These variables are set by the <stat> function (here), and contain various information about the file(s) requested:

$ret.err, $ret.ownerid, $ret.owner, $ret.groupid, $ret.group, $ret.size, $ret.isrd, $ret.iswr, $ret.isex, $ret.mode, $ret.atime, $ret.mtime, $ret.ctime, $ret.depth, $ret.symlink, $ret.sympath, $ret.nlinks, $ret.devtype, $ret.dev, $ret.ino, $ret.blks, $ret.blksize, $ret.attrib

In syntax version 8 and later, $ret.size is also set to the int64 file byte size of the entire line, by <readln> (here). The returned size includes the length of the (unreturned) EOL character(s), if any.