Umm something like
find /start/directory -name *.DAT -type f -size +5M -print | xargs /bin/rm -f
might find the files (test it first by only taking in the 'find' part, i.e. don't write '-print' nor anything after it -- if it lists correct files, then use the whole line). That's just off the top of my head, so be aware. Read
for more instructions.