4,300 happens to be small enough to have int(s) in under 100 usec on modern hardware and large enough to pass unit tests of popular projects that were in our internal test sytems. My initial value was 2,000, which is easier to remember and more than large enough for a 4k RSA key in decimal notation. We later bumped it up to 4,300 because the test suites of some popular Python projects like NumPy were failing with 2,000.
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