What's the lowest optimal sample rate for audio recording on iPhone while still yielding good file size? Currently I am using 44.1kHz but it produces file that's way too big.
It depends on what you mean by "optimal". 22.05kHz would still be good enough for most music. 11.025kHz would be more than adequate for recording speech. It all depends on what you're doing with the audio. I recommend using your ear to determine the best rate.