According to wikipedia:
The cryptographic strength of the HMAC depends upon the size of the secret key that is used.The HMAC RFC in turn states that:
2. Definition of HMAC
....The definition of HMAC requires a cryptographic hash function, which we denote by H, and a secret key K. We assume H to be a cryptographic hash function where data is hashed by iterating a basic compression function on blocks of data. We denote by B the byte-length of such blocks (B=64 for all the above mentioned examples of hash functions), and by L the byte-length of hash outputs (L=16 for MD5, L=20 for SHA-1). The authentication key K can be of any length up to B, the block length of the hash function. Applications that use keys longer than B bytes will first hash the key using H and then use the resultant L byte string as the actual key to HMAC. In any case the minimal recommended length for K is L bytes (as the hash output length). See section 3 for more information on keys.
So that in effect means that our secret key should be 16 bytes for MD5, 20 bytes for SHA1 and larger if you use SHA-2 or SHA-3. Use the output bits column of this table to figure out what your secret key size ought to be. For the flask secret key size, I believe that a 32 byte secret key should be sufficient (and a 16 byte secret key risky... :)3. Keys The key for HMAC can be of any length (keys longer than B bytes are first hashed using H). However, less than L bytes is strongly discouraged as it would decrease the security strength of the function. Keys longer than L bytes are acceptable but the extra length would not significantly increase the function strength. (A longer key may be advisable if the randomness of the key is considered weak.) Keys need to be chosen at random (or using a cryptographically strong pseudo-random generator seeded with a random seed), and periodically refreshed. (Current attacks do not indicate a specific recommended frequency for key changes as these attacks are practically infeasible. However, periodic key refreshment is a fundamental security practice that helps against potential weaknesses of the function and keys, and limits the damage of an exposed key.)
My secret key block then becomes:
SECRET_KEY = open("/dev/random","rb").read(KEY_SIZE)
os.urandom from the stdlib may not cut it since it sources /dev/urandom.
Caveat: YMMV, I am not a security expert!