By default wordpress often displays a smaller version of your image – So don’t be alarmed if you try inserting an image into your description and the image is tiny 🙂
When you upload an image, there is the option to select what size it will display as on the page (The actual measurements of the smaller sizes will depend on the original size of the image). By default it’s usually set to display at a medium size. So you can make sure it is full size when you first add it.
If you’ve already got a tiny image on your page, don’t worry, you can still resize it. Click the image and you’ll see a menu pop up that gives alignment options and a pencil (edit) button. The image should now have little squares on each corner.
If you put your mouse pointer over one of the corner squares, you can hold your left mouse button down and drag the corner of the image to resize it to the size you want. At first this may make the image blurry, but it should update properly.
Or, you can click the pencil, which will bring up the “image details” window – here you can set it to full size, make it into a link and other options.