So where are you stuck ? Also, make it clear, do you want to search some particular files or want to serach some keyword in the file content?
You can use -name
option, instead of grep piped with find cmd, like:
find $wheretosearch -type f -size -800k -mmin -1440 -name "$phrasse" -print >> $outputfile
Don't forget to close for loop in your script with do-done.