I can see from the details of your screen shot that this is a windows system and will guess that you had windows maximized when you ran this script. It is a unfortunate issue with FileMaker that when you open a new window, FileMaker drops out of the windows maximized state (I could live with that) and then resizes the parent window to whatever window size was last specified when the windows weren't maximized (that's why your original window changes to a completely different size)--that's a "feature" I could do without.
Either keep your windows out of the windows maximized state or use this code to minimize the window resize triggered by the New Window script step:
Select Window [get ( FileName ) ] ---> one way or another select the original window
Resize Window [ ]--Specify a window size as close as possible to the original maximized size, you can use get (windowheight) and Get ( WindowWidth) to specify such dimensions for your original window.
Then you'll need to reset windows to "maximized" at the appropriate time such as when you close the new window that you have just created with the new window command.
Change "Current Window" To Floating Document and besure current file is checked.
Thanks for both responses. That helped