CBC MAC that is using AES as the underlying block cipher, has tag space of 2^128. If we add a random single block, concatenating it with the tag, and encrypt it again in AES CBC mode, with additional key, we get a 256 bit tag of 256 bits space:
TAG = CBC_MAC(K2,CBC_MAC(K1,M)||r) || r
(where r is 128 bit random number) This is a randomized MAC generating tags in the space 2^256. In the same way we can easily generate tag in any size. Is there any standard that is using this method?