Resizing image before adding to word document is essential thing to make the document size smaller. If you resize a large image of 5MB inside word document, it won't help you make the document size smaller. Imagine having lots of such images in a single document. If you use some basic image editor to decrease size and resize the image as per need, then add to word document you will have a it in small size. You won't have much problem while sending lots of such documents in email. You will save a lots of internet cost if you are on a costly connection.