I guess it also depends on how you intend to smoke it, and how strong you made it.
With larger pieces, completely burning one takes longer than with smaller pieces. If the material is very strong or the pieces very large, so that a dose is just a few of them, this could mean that towards the end of the inhale, only a few spots are still glowing, and more air passes where there are none, diluting the smoke and prolonging the inhale even further.
With very fine pieces, you get more of one big ember, producing a relatively constant stream of smoke throughout the inhale. However the opposite effect might occur, where the material is so dense that air hardly gets through and most of the smoke escapes, especially if the changa is rather weak or the particles very fine, so that a dose is a ton of them. Also at some point you risk pieces getting through the filter/screen into your mouth or even lungs, which is very unpleasant (that's where a bong is really nice - the water catches any pieces, embers and ashes potentially making it through). You definitely don't want it as fine as dust, unless maybe if your method of inhalation is to place it on a piece of hot coal in an inhalation chamber.
For most smoking devices and my changa that is 40%-50% DMT by weight, i like it to be in pieces of about 2mm to 5mm length. Of course if they're too large, i can just break them up in preparing the bowl. If they're too small, i might have to mix it with something more coarse to make it work. With weaker changa, i'd want it to be a bit coarser, say 4mm to 7mm, so i still get good airflow with a larger heap of material.
Whatever smoking device you'll use, you get the most out of your dose if you can inhale directly into the lungs, and keep it there for a while before exhaling (longer = stronger, within reason). My tip to reduce the risk of coughing is to pull the smoke into the stomach first then fill the lung upwards, and to always inhale some pure air after the toke (remember to leave space for that, or remove the pipe/joint from your mouth a second before you run out), so that there's air and not smoke in the throat and upper chest while you hold. You want the densest smoke you can tolerate, to ideally get the whole dose in one lungful. Slow and steady. By the time you exhale, the smoke should be rather thin.
With changa you don't have the same timing issues as with pure DMT though, if it takes you 17 smaller tokes, that's fine. If you feel like taking another toke 10min in, that's fine. Slower intake just means more gradual onset and requires a bit more material for the same effect.
