When adding a watermark to the word file, by default, the size of the watermark is pretty large. If you want to re-size it to a different size, please follow the steps below for details:
Step 1: Double click the header of the file and move the cursor in the header;
Step 2: Move the cursor on top of the watermark, and click on it when the cursor turns into a black cross;
Step 3: Gray dots appear around the watermark. Move the cursor on one of them and drag to re-size the watermark. The square dots in the middle are for width or length and the round dots in the corners are for both width and length together.
Alternatively, you can use the method below to create an exact size of the watermark.
In Step 3 above, instead of clicking and dragging the watermark to the proper size, you need to double-click on one of the gray dots. The "Format" tab will appear in the ribbon when double-clicking the gray dots.
In the "Format" tab, change the numbers of the height and width to get the exact size you need.
Please check how to change the angle of the watermark in word if you need to change the watermark angle.
If you have a small size of watermark now and want to change the location, please refer to how to change the watermark location.