int contentLength

The size of the request body, in bytes.

This defaults to null, which indicates that the size of the request is not known in advance.

Source 

int get contentLength => _contentLength;
void contentLength=(int value)

Source 

set contentLength(int value) {
  if (value != null && value < 0) {
    throw new ArgumentError("Invalid content length $value.");
  }
  _checkFinalized();
  _contentLength = value;
}