You're going to be slicing the carrots for both the purée and the coins for the top. Keep in mind that the size of the slices will affect the cooking time. For the purée, just be sure that they're cut uniformly, so they cook evenly. For the top of the crostata, 1/4 inch slices are a good size. Remember that they'll shrink as they cook and really thin slices will burn around the edges.