Searching for "small blocksize cipher" finds a number of discussions on the topic, mostly refering to FPE.
This one in particular suggests using AES as the round function of a Feistel network.
The problem is on some embedded devices, the use of AES is too complex, so a tradeoff between cipher strength and complexity is required.
In searching for a Feistel based cipher with low complexity, the TEA cipher seems a good choice, but uses 64 bit blocksize.
For a sub 64 bit block of data, say 44 bit, tweaking TEA by masking the output of each Feistel round with a 22 bit mask (0x3FFFFF) seems to work correctly.
The question is, does masking in this way break the TEA algorithm? (other than weakening due to a smaller blocksize)