Here's my take, based using different sizes of smoke woods in various smokers: smaller wood = more surface area per amount of wood = faster ignition and shorter smoke times; larger wood = less surface area per amount of wood = slower ignition and longer smoke times. That said, if you want the smoke to start early and run long, use small and medium chips, and small, medium and large chunks in combination.
How and where you load your smoke wood in conventional smokers will also determine how soon smoke will start, as well as how long it will last. The hotter your smoke wood gets, the faster it will smolder (hopefully not burn). Being that larger chunks smoke slower, they tend to provide less smoke flavor, so, to offset this, using the combination of various sizes should give better results. If you place smoke wood directly on the heat source they will smoke more, but regulating the heat and keeping them from burning by reducing the air getting to them isn't always easy. If enough heat is present and too much air is present, they simply catch on fire and spike your smoke chamber temps...and can add some soot to your food as well.
Smoke wood size combinations seem to work best for most everything I've hot smoked where I'm not using a charcoal smoker, though with lava rocks, chunks may work fine for you. As far as how much smoke wood to use is a personal preference. If you like a lot of smoke, smoke it until it comes out to rest before pulling...if you foil to reach finish temp/texture, smoke won't really matter after foiling. I will say with chunks, you won't add smoke wood nearly as often as with chips, though I have had 4-6 hour smokes with chips in my propane smokers in the past when using good methods for controlling the rate of smolder, but chips for a heavier and faster smoke up front, and chunks to keep it coming for hours afterwards is a much easier way to go.
Sorry I can't give definitive answers as to how much wood and where to place it, but sometimes you'll need to just experiment a little to find out what works best for your personal preferences on desired smoke flavor, and, what your smoker likes based on it's configurations and mods. Any mods you do, or even adding heavier loads of food, will change how your smoker operates, usually much more than we realize.
Eric
