There are two possibilities to prevent users from resizing custom windows:

Option 1) Through the form's properties:

You need to use the following properties for the form that will be displayed in the window:
Sized based on: Set size
Resizable: Yes
Maximum and minimum width and height need to be identical.

If you choose this method, it will apply only to the window when that specific form is being displayed. If, for example, you set it for an output form and users have the possibility to display records in the input form, make sure both form properties share similar options.

Option 2) Through the window type:

When you open the window through either Open window or Open form window, you can choose a non-resizable window type (4). Window types are described in the 4th Dimension Language Reference in the Chapter "Windows".

If you choose this method, the size of the window will remain constant, no matter which form is being displayed.