If you set a 500 character password it would still be possible to crack, plus there are vulnerabilities in excel that instead of brute forcing a password you can simply change the password to something different without ever needing to know the original password.
You might be better of to set up a network folder that only you have write privledges to and everyone else only had read privledges for.
well, when i distribute the file to people, they can easily use a excel password cracker and get into the locked cells and look at the hidden data, etc
Excel security isn't designed to stop people looking at the data, more to stop accidental changes. Anyone with any significant knowledge can bypass both sheet and VB password protection
If you have sensitive calculations then copy and paste special values into a copy of your sheet