Resampling 48kHz audio to 16kHz
If you are going to a 16kHz sample rate, anything equal to or more than half that will cause aliasing. So, by 8KHz the attenuation of the filter needs to be high enough that the aliasing is acceptable.

What order and cutoff to use is up to you. A higher order will allow the passband to get closer to 8kHz since the transition is sharper. A lower order is simpler.