Big Blue Saw has done quite a bit of waterjet cutting of small parts. Small parts are often jewelry made with special materials like silver and bronze.
Let's take a look at how this can work out for a simple trefoil design cut from 1/8" thick aluminum plate. As you can see below, we made the design in several different sizes, from about 2 inches (50 mm) across down to 1/4 inch (6.4 mm) across.
The parts all had a rectangle at the top 0.125 inches wide. On the smallest two pieces, the lobes were 0.262 (6.7 mm) and 0.131 inches (3.3 mm) in diameter.
You can see how they turned out in the closeup photo below. Note that our limiting factor here is the cutting stream size (kerf width) of the waterjet: about 0.04 inches or 1 mm in diameter. That's slightly thinner than a CD or DVD. You will notice that the sharp inside corners of the original design become visibly rounded off at this scale. Designs at this scale and smaller are quite limited in the amount of detail they can have.
Below is an even closer look at the smallest piece. The top lobe is slightly asymmetrical, possibly due to the waterjet cutting path or vibration during cutting.
Small parts are typically tabbed to the sheet from which they're cut to prevent them from falling into the water catch tank. Here we can see that the tab is about 1/64 inch (0.4 mm) wide. This is almost too small for this particular design and material. The larger pieces actually broke loose from their tabs during the simple handling needed to take these photos. The tab is designed to be thin enough that we can remove the part from the sheet easily. (Thin areas tend to break.) Thinner and weaker materials (remember that we're using 1/8" aluminum here) will likely require a wider tab, which could interfere with a small part's design.