If you use scripts with the new window command to open the new windows, you can control where, within the application window, the new window is located and it's size. These top, left, width, height settings can be calculations that draw on the location and dimensions of an existing window to make it's location relative to the first window's size and postion.
I often use this method to be sure a small "psuedo-dialog" window opened with this script step is perfectly centered on either the monitor screen or the parent window.
To capture window or monitor dimensions, check out the Get functions in FileMaker help. You can use these functions to capture the needed data in variables before you open the new window and then refer to them in the above mentioned size and location calculations to control the window size and location.