Controlling default image sizes

Whenever you upload an image, WordPress provided three options for resizing of the image (aside from retaining its original size): Thumbnail size, Medium size, and Large size. You can set the pixel dimensions of these options by going into Settings | Media from the main sidebar menu of the WordPress dashboard. This will take you to the Media Settings page:

Here, you can specify the size of the uploaded images for the three resizing options we just discussed.

If you change the dimensions on this page and click on the Save Changes button, only the images you upload in the future will be affected. The images you've already uploaded to the site will stay at their original/previous thumbnail, medium, and large versions.

It's a good idea to decide what you want your three media sizes to be early on in your site's lifespan, so you can set them and have them applied to all images, right from the outset.