Just make sure the vegetables are cut to be approximately the same size, whether 1/2" or 3/4". This helps to cook everything evenly. If some are smaller, they get done too fast, if some are bigger they will be too firm. Other than that, choose the size you'd like. BB

It looks like you are referring to the Alice Waters' Ratatouille recipe.

If you look at the photos, slide #3 shows the person cutting eggplant into 1/2" dice.

You are free to change this if you want chunkier ratatouille. This dish does not require extreme precision in the vegetable piece size.