Its in right-click on source->Transform->Edit Transform
If the Bounding Box Type is set to "No bounds", the size of the source is determined by the size values of that dialog and the source is resized proportionally if the captured window changes its size.
If the Bounding Box Type is set to something else, the size of the source is determined by the Bounding Box Size and depending on the exact entry of Bounding Box Type. The source is zoomed automatically to match those two settings.
Easiest way to get a fixed source size and automated zoom to match the given size is to right-click source->Transform->fit to screen, then resize the now fullscreened source to the desired size by dragging the borders in the preview with the mouse. However, if the captured window is resized to a different aspect ratio, the source will get resized to match the changed aspect ratio. To always match and fill the given size and ignore aspect ratio, use stretch to screen instead of fit to screen, but this will distort the picture and is usually not what you want.
To get back the auto-resize behavior, right-click source->Transform->Reset Transform.