If you have an image that isn't the desired size, on your page or post, and it must be adjusted.

Login to your WordPress admin area.

Click on Pages or Posts depending on which you want to edit.

Click the edit under the page or post title.

Click the Add Media button.

Select the image you wish to insert.

Under the Attachment Display Settings on the right, select the desired size from the Size drop-down.

Click the Insert into page button.

You have now updated the size of the image you are working with.