There are several ways to resize a picture. An image's file size is largely determined by its resolution and dimensions. Typically changing one, will change the other.
1. Right click on the picture file and select Open With > Paint.
2. In the Home tab, in the Image group, click Resize.
3. In the Resize and Skew dialog box, in the Resize group, ensure Maintain aspect ratio is ticked so the relative width or height remains the same.
4a. To resize your picture by a certain percentage, ensure Percentage is selected and then enter the new values in the Horizontal OR Vertical boxes and click OK.
4b. To resize your picture to specific dimensions, click Pixels and then enter the new values in the Horizontal OR Vertical boxes and click OK.
5a. To Save a Copy of the image click File > Save As. Type a new file name in the File name box, and then click Save.
5b. Alternatively, if you do not wish to keep a copy of the original image. Click File > Save.