Spurgeon had advice for pastors on the length of their message. He used the illustration of bread.
Do not overload a sermon with too much matter. All truth is not to be comprised in one discourse. Sermons are not to be bodies of divinity. There is such a thing as having too much to say, and saying it till hearers are sent home loathing rather than longing. An old minister walking with a young preacher, pointed to a cornfield, and observed, “Your last sermon had too much in it, and it was not clear enough, or sufficiently well-arranged; it was like that field of wheat, it contained much crude food, but none fit for use. You should make your sermons like a loaf of bread, fit for eating, and in convenient form.”
-C.H. Spurgeon, Lectures to My Students, p. 77