Hi Keith Baldwin,

Yes you can, but not from the Window designer
You can set the trim width from the properties bar
Draw your wall--> insert your window--> with the window selected--> properties bar (right of the screen)--> set the dimensions and the trim width--> Don't forget to press enter in your keyboard after a change in the properties bar

If you set the width as 4" (for example) as well as other windows' dims and elevation, these values will remain as default during all the project and each time you select a new window to insert, the properties bar will show these values, unless you change them. But if you wish to change one trim to 6" (for example), just select this particular window, change the trim width to 6" (Properties bar)--> enter --> your new window is ready

Let us know if you need additional help

Patricia