function humanSize (size, opts) {
  if (typeof size === 'undefined' || size === null) return 'N/A'
  return filesize(size || 0, {
    // base-2 byte units (GiB, MiB, KiB) to remove any ambiguity
    spacer: String.fromCharCode(160), // non-breakable space (&nbsp)
    round: size >= 1073741824 ? 1 : 0, // show decimal > 1GiB
    standard: 'iec',
    base: 2,
    bits: false,
    ...opts
  })
}
@param {number} size in bytes
@param {object} opts format customization
@returns {string} human-readable size