Welcome. There is no problem with app acceptance and OpenEars, it has public APIs and only uses public APIs. This is covered in the FAQ but you can also see several well-known apps that use OpenEars to the right of the main OpenEars page at https://www.politepix.com/openears so you don’t have to take my word for it.
Reducing your application size is also covered in the FAQ so give it a read:
OpenEars should only increase your app size by about 7 mb if you are using speech recognition and maybe another 5mb per voice used for TTS.
By the way, the App Store doesn’t limit your app size to 20mb; there are apps that are hundreds of mb in size. But I agree that it is friendly to your users to create the smallest app possible.