Like the others have said... follow your nose rather than your eyes.
You don't need a big piece of wood to get a good smoke, in fact, when it comes to making "thin blue smoke", more is NOT necessarily better. The key is to have small amounts of wood burn efficiently, so you get the sweet smoke flavor, and not a bitter, over-smoked flavor.
Sweet blue can be seen but you have to get the right angle of light on it. It's very thin, and wispy, and almost completely invisible.
If you smell wood smoke, leave it alone, if you do not then add a few chunks, right on top of some red-hot coals, the best smoke is smelled, not seen.
Humphrey's DownEast Beast W/BBQ Guru