Do you mean the file size or the image size? You can resize the image by
selecting the image, then click and drag the sizing handles (the little
black squares at each corner and in the middle of each side). Or right click
the image, select 'Format' from the popup menu, and set the size explicitly.
But in general it's better practice to size the image using a graphic editor
(and set the resolution) before you bring it into Word.