How do I change the default height of a BottomSheetDialog?

You can set a bottomSheetDialogTheme in your Activity, overriding the bottomSheetStyle attribute’s behavior_peekHeight:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
  <item name="bottomSheetDialogTheme">@style/AppBottomSheetDialogTheme</item>
</style>

<style name="AppBottomSheetDialogTheme"
       parent="Theme.Design.Light.BottomSheetDialog">
  <item name="bottomSheetStyle">@style/AppModalStyle</item>
</style>

<style name="AppModalStyle"
       parent="Widget.Design.BottomSheet.Modal">
  <item name="behavior_peekHeight">@dimen/custom_peek_height</item>
</style>

This same technique can be used for other attributes as well, such as adding <item name="behavior_hideable">true</item> to the AppModalStyle to change whether the bottom sheet is hideable.