My rule of thumb is that given an idea of a particular feature, i.e. a specific page or service, you should be able to find the file its in through auto completion alone.
If you lack inspiration, here are some folder structures I use:
theme/buttons.dart theme/text_input.dart theme.dart auth/auth_service.dart auth/auth_impl.dart utils/color_utils.dart utils/state_utils.dart utils.dart common/potato_card.dart common/foo_widget.dart common.dart profile/profile_service.dart profile/profile.dart profile/user.dart profile/member.dart profile/guild.dart profile.dart pages/home/home_page.dart pages/home/cards/foo_card.dart pages/home/cards/bar_card.dart pages/home/foo_dialog.dart pages/settings/settings_page.dart
I like to keep it as flat as possible, avoid creating categories that don’t add meaning to a particular file.