Well, yes, there is. When exporting a diagram, yEd shows a dialog with format specific options. On the first tab, there is an option "Size". Its default value is "Original Size". Change that to "Custom Width" or "Custom Height" and specify the desired width/height (in pixel) as well.
However, when exporting a diagram to a raster image format (like PNG), a much better approach is to export the diagram with "Original Size". Before importing the exported image in Word, open the image in a image editor application like Adobe Photoshop or The Gimp. Change the image resolution from the default 72dpi to a resolution that ensures the image fits into your page.
To do that, you need to calculate the required DPI using the following formula:
(image width in pixel)/(page width in inch),
(image height in pixel)/(page height in inch)