Gets or sets how many bytes to reserve in the signature's Contents for the token, which is only known once it arrives,
after the byte range is fixed: 8192 by default, enough for an authority certificate and its issuers. A token that does not
fit throws InvalidOperationException and nothing is written; raise this for an authority with a long chain.
public int TokenSizeEstimate { get; set; }
ISO 32000-2 §12.8.1, Table 255, Contents: "Space for the Contents value shall be allocated before the message digest is computed".