If you must use scanf then I believe that the best that you can do is use the width specifier with something like: "%31s", as you've already mentioned, then use strlen to check the length of the input, and discard the string and report an error if the input is longer than your limit.
Or possibly skip the strlen by additionally using an %n in your format string, e.g. "%31s%n".
A format string using something like %[^
] in place of %s simply instructs the function to continue reading until a newline, consuming other whitespace characters along the way. This is useful if you want to allow the input to include whitespace characters.