You're working in the 8.0 beta, right? Would using the "Set Size" action with an anchor point set to "top left" do the trick for you? That idea could work if the widget that you want to resize is sitting directly below the header that you don't want to have overlapped.
If you have a specific size that you want to set the widget to then you can specify in the "width" and "height" fields in the case editor. If you want it to do something else like fill up the entire width or height of the browser, then you can click the "fx" button next to the width and height fields to add functions that will let you do that. If you want it to fill up the entire width of the browser, you can set it to [[Window.width]]. Similarly for filling up the window height, you could set the height to [[Window.height]]. Those functions are available in the "Window" section of the "Insert Variable or Function" list.
If you want to do something like make the height of a widget match the height of a browser but now cover up a header, then you can set the height to something like [[Window.height-(your header's height)]]. For example, if your header is 100 pixels tall, you would put [[Window.height-100]].
If the height of your header is going to be changing sizes too, then you can set up a local variable in the function editor that corresponds to your header. For example, LVAR 1 == your header widget. You could then write something like this:
That should allow your widget to adjust to the height of the browser minus whatever the height of your header becomes.
Variables can get a little crazy. Hopefully that made sense!