Subkey generation in OMAC1/CMAC with XTEA?
Look at how the keys $K_1$ and $K_2$ are used in CMAC (pdf, Section 6.2): 


  
  If $M_n^*$ is a complete block, let $M_n = K_1 \oplus M_n^*$; else, let $M_n = K_2 \oplus (M_n^*||10^j)$, where $j = nb-Mlen-1$.
  


They are combined with message blocks using XOR. So they must be equal in length to the block size, not the key size (if different), of the cipher.

Similarly, then, $C$ is determined by the block size.