If you know the Regular Expressions then you may try to change settings of the Password field via Developer->Forms->System->Account->Create account area. The "Password" field has the following options provide this:

Check for with the value "Regular Expression (something specific must be entered)" and Expression with the value 

~^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}~

It means that the field should have some lowercase and uppercase letters, digits and length not less than 8 symbols.

For start you may try to change 8 to another number and press "Save" button then "Password" field will require another length.