There isn't any magic "fit to screen" feature in FileMaker. There are a number of tricks and techniques that sort of work part of the time.
There's a Resize To Fit option in the Adjust Window script step that can pop your window out to the limits of your layout.
You can use auto-size anchors to set layout objects to slide or stretch to the right and/or down as the window size increases. If you select top and bottom anchors for a portal, taller displays will show more portal rows than shorter as it adjusts to fit the display dimensions. (This can be very cool way to make one layout adjust to an iOS device's current orientation.) And I sometimes put a small invisible layout object such as a rectangle in the lower right corner of the layout and select only the bottom and right anchors. This will automatically expand the layout out to the limits of the window used to display it.
If you remove all anchors for an object, it will tend to 'center' within a layout. As you increase window size, it will retain it's relative position on the layout such that the empty space created by the window increase in size is evenly divided on opposite sides/top and bottom of the window. I use this method to keep all the controls on a "main menu" layout properly centered for small differences between computer monitor dimensions.
In Windows, you have a preferences option that will "enlarge window contents to improve readability".
You can set up scripts that change zoom settings for different users and your OS has display settings that can change how things display as well.
And you can set up a "display options" table where you specify details for each user's device to select the best combination of zoom settings, window sizes and layout sets that best fit their device's display--mix and matched with OS display and FileMaker preferences settings to get the best result possible.
This all sounds rather complex in comparison to my dream of a 'magic button' :)
I had best get to work changing my layouts.......