code: func (c *ServerContext) Sizes() (uint32, uint32, uint32, uint32, error) {
  	return c.sctxt.Sizes()
  }
docs: // Sizes queries the server context for the sizes used in per-message
  // functions. It returns the maximum token size used in authentication
  // exchanges, the maximum signature size, the preferred integral size of
  // messages, the size of any security trailer, and any error.