You start by defining a goal for your design, usually in antenna performance terms including gain, efficiency, bandwidth, frequency. If you then decide that a microstrip array is best suited, the main considerations will come down to, as you point out, the number of elements and the spacing between them, and in addition, whether the elements are all in one plane, and most importantly how they will be fed. Will you feed them in phase so to form a broadside array, or something else? The specific shape, size and thickness of each element will have the most effect on bandwidth and center frequency. The type of substrate will have strong effect on efficiency.