Unfortunately, Windows does sometimes (but not always) allow you to resize Plant Tycoon's window. However, since the game is designed for a fixed screen size, making the window larger (or smaller) will throw off the game's calibration and produce the problem that you saw. This is not something that is easily fixed, and we recommend that you don't resize the window. The only way to easily restore correct screen calibration is to close the game (using the 'X' in the upper right corner) and launch it again.