To track and display download progress you can use the OnDownloadProgress event of the HTTPRequest class. This event’s parameters are the original HTTPRequest object, the downloaded bytes and the expected length of the downloaded content.

var request = new HTTPRequest(new Uri(address), OnFinished);
request.OnDownloadProgress = OnDownloadProgress;

void OnDownloadProgress(HTTPRequest request, long downloaded, long length) {
	float progressPercent = (downloaded / (float)length) * 100.0f;
	Debug.Log("Downloaded: " + progressPercent.ToString("F2") + "%");