This is based your CPU and GPU architecture. 1 Tile = 1 Thread. Example, if your CPU has 8 thread and you set tile size 32 x 32; 32 * 8 = 256. Your CPU can render 256 x 256 area at a time.
When use GPU render, generally GPUs can only 1 tile render at a time. For this reason, you must use big tile size. For most GPU, best value is 256 x 256.