"For who has despised the day of small things?" No one who understands God and what He is working out looks down on the times when only insignificant things seem to be accomplished. Those who understand what God is doing know that the day of small things must take place before the big things can happen.
This is primarily an encouragement to the Two Witnesses. Their work will appear as nothing to begin with. Nevertheless, they will not despise it because they know that small things must happen before bigger things can take place, the things that will really put them on the map during the final 3½ years. But the small things that happen before that time—in measuring the altar, the Temple, and the worshippers (Revelation 11:1)—will set the stage for their major work.
It is important to realize—from the historical point of view—that even when this Temple was finished, the people moaned about it: "This is nothing like Solomon's Temple!" It seemed a small thing in itself, and it was. It was just a bare representation of the original Temple that David built through Solomon. Nevertheless, it was necessary. The small things that happened back then—the Jews returning from exile with a great many of the Levites and the priests, building the Temple, putting a wall around the city, and eventually colonizing most of the old land of Israel (particularly around the Sea of Galilee)—made the birth and ministry of Jesus Christ possible. He had to have a Temple to come to.
So, all these small things that happened with this tiny number of people who came back from Babylon, and all the work that they did over a hundred years or so, prepared the way for the very "big thing" of the first advent of Jesus Christ (meaning His entire life, His ministry, His death, and His resurrection). Without the small things, that big thing would never have happened.
God was preparing for the big thing through the small things, and He does that all the time. Thus, any faithful person will not despise the times when only small things are happening, because they mean that big things are coming and that they should prepare themselves for them.