How to validate password length in wordpress

if ( strlen( $_POST['pass1'] ) < 3 )
{
   //passwords so small
   $redirect_url = home_url( 'member-password-reset' );
   $redirect_url = add_query_arg( 'error', 'password_reset_mismatch1', $redirect_url );
}

I see here that you’re checking if password length is less than 3. I guess you have to change 3 to 8:

if ( strlen( $_POST['pass1'] ) <= 8 )
{
   //passwords so small
   $redirect_url = home_url( 'member-password-reset' );
   $redirect_url = add_query_arg( 'error', 'password_reset_mismatch1', $redirect_url );
}

Is that what you want to achieve?