Small and low energy enough that tiny migratory birds can wear them for months. Externally worn of course (e.g. attached to the ear, for a wolf).

You could adjust the firmware of a wildlife tag to start transmitting location every 10 minutes when the animal leaves a geo-fence.

size of chip? they're tiny. dog owners typically have the vet "chip" their pet as a puppy. full-grown dog doesn't need a bigger chip.