1. The first option is to click on the image you’ve placed in your post’s edit window then click on the little pencil icon. In this window you can select from 4 options in the Size pull-down menu: Thumbnail, Medium, Full Size or Custom.
2. If you select Custom, you can enter the size you want assuming it is smaller than the original.
3. You can also click on Edit Original and scale the image from the original size.
4. You can edit a WordPress image from the Medial Library. Just click on Edit under the image in the library then click on Edit Image under the large image version that appears.
5. The final way to resize an image is to click on it in the post or page edit window and drag the bottom right corner toward the center (or away from the center if you want to enlarge it to not more than the original size).