BAEssentiaMaxInstallSize / BAMaxInstallSize should be the size that your files take up on the user's device. So if you're extracting files, you'll want to use the extracted size. The value you place in these keys will be shown to the user on the App Store.
BAEssentialDownloadAllowance / BADownloadAllowance are restrictions against your extension that control how many bytes can be downloaded before the app has been launched. That's why these keys are in a dictionary named BAInitialDownloadRestrictions; since they're only initially enforced.
These essential keys are completely separate from the non-essential keys. The essential keys should be for BAURLDownloads that have the isEssential property set to true.