result.txt

sendBeacon attempt 0 with 0 bytes succeeded
...
sendBeacon attempt 362 with 65703 bytes succeeded
sendBeacon attempt 363 with 65703 bytes failed

script.js

let bytes = 0
for (let i = 0; i < 1000; i++) {
  const payload = Array(i).join('a')
  const result = navigator.sendBeacon('http://localhost:9000', payload)
  if (result) {
    bytes += i
    console.info(`sendBeacon attempt ${i} with ${bytes} bytes succeeded`)
  } else {
    console.warn(`sendBeacon attempt ${i} with ${bytes} bytes failed`)
    break
  }
}