Questions !!

How do you determine the array size for a microstrip planar array ? How do you know how many elements you need in the x and y directions ?

It depends on angle of scanning and most of all on the desired main beamwidth. You can find some information about this theme in Antenna theory from Balanis (you can find this book on this site by using search)

Yes i know. But for microstrip antennas, most of the time the scanning angle will be either 0° or 180° since they give rise to an end fire radiation pattern. Some time ago, i heard someone told me that by altering the size of the antenna array, you can actually still get the same beamwidth. How is this possible ? What are the variables that exist that alters the array size without changing the beamwidth ?