Certificate validation on an IoT device using few bytes
Generally you'd let the device establish a symmetric / secret key first, e.g. using ECDH and then use that secret key with a MAC algorithm to perform the message authentication. In that case 16 bytes / 128 bits is plenty.

For direct signatures, the BLS signature scheme is probably still the best out there, but for 128 bit level encryption - the lowest we commonly aim for - you'd still need a 384 bit signature.