For stews, aim for roughly uniform slices, about 1-inch thick, to ensure even cooking and consistent texture. Slight variations are acceptable, but avoid drastically different sizes.
The key to a great stew is even cooking. Root vegetables like carrots, potatoes, and parsnips take longer to cook than other ingredients. If the slices are drastically different in size, the smaller pieces will become mushy while the larger pieces may still be undercooked. Aim for pieces that are approximately the same size, around 1-inch thick. This allows them to cook at a similar rate, resulting in a stew where all the vegetables are tender and cooked through without being overdone. While perfect uniformity isn't necessary, consistency is important. A little variation is fine and can even add visual appeal, but avoid extremes. Consider the density of the vegetable as well. Denser vegetables like carrots might benefit from slightly smaller pieces than less dense vegetables like potatoes.
Cut your root vegetables slightly larger than you think you need. They will shrink a bit during the long cooking process of a stew, so starting with slightly larger pieces will help them maintain their shape and texture.