Changing Image Size in Post

I have loaded images into my media library that are 520 pixels on the long side. However, when I add images to a post they are only 300 pixels long. I can in advanced settings change the sizing of any one dimension but cannot get the image to enlarge proportionately. I am interested creating a photo blog and would like better control of how my images post. What is the easiest way to resize images in posts so that the changes are proportional? Or how can I change the settings so that any images I post stay at say 520 pixels as opposed to 300 pixels?

Hugh