To reset password of specified length in Membership provider

for the following you need to set the allowpasswordretrieval property to "true" in web.config in SqlServerMemberShipProvider  

using System.Web.Security;
MembershipUser user = Membership.GetUser(userName);
// to get current password
string oldPassword = user.GetPassword();
// to generate a password of 10chars with 4 special chars
string newPassword = Membership.GeneratePassword(10, 4);
// ResetPassword generates a password of 14chars, so we use changepassword.
user.ChangePassword(oldPassword, newPassword);