Cheers, big ears.

If some one has good hearin do you say they have ears like a bat or is it something else

In the UK if you use 'bat ears' about someone it means they have large ears - not very complimentary. In this sense it has nothing to do with hearing, only size. It's a term which is even used medically to describe the condition, or was in my day!

On the model of "eyes like a hawk," which would be eyes with really extraordinary vision, "ears like a bat" may signify ears that hear exceptionally well, as a bat's ears do. I'm not sure that it's appropriate for humans, since most bats hear only frequencies too high to be heard by humans.
SS

I think bats' ears are so often associated with large ears that "ears like a bat" would be understood in the same way as "ears like Dumbo the elephant". "Blind as a bat" is often heard, although bats don't actually have bad eyesight. Pamela