That's an interesting question. Some quick searching gives me these figures:
* Knowing 10,000 words in a given language is considered the level required for fluency
* A standard speaking speed is 130 words per minute, or 7800 words per hour
* At 256 kbps, a minute of music takes 2 MB of storage, or 120 MB for an hour.
Let's say the app contained a professional recording of 7800 words (which should be more than enough for teaching just pronunciation and intonation), and let the user record up to 4 attempts at each word (e.g. their first attempt, their latest attempt, and a couple of intermediate attempts). That would mean 5 copies of roughly an hour's worth of speech, so 600 MB of storage. For comparison, that's about 1 or 2 minutes of iPhone 4K video.[0]
Anyway, thanks for giving it consideration, and I'm sorry I don't know anyone's who's into marketing. I wish you every success with the app!
[0] https://www.macxdvd.com/mac-video-converter-pro/4k-video-fil...