Modifying post image sizes using this method requires editing posts in the Edit Html mode. Blogger's small editing window doesn't make it easy, but it's not difficult – just work carefully.
When images are uploaded using the post editor's image button, Blogger stores the original (max of 1,600px on the long side) and produces a scaled copy for display. The sizes are: small = 200px on long side; medium = 320px on long side; large = 400px on long side.
The size is included in the <img src> code as a directory or folder name. /s200 is for small; /s320 for medium; /s400 for large, and the the width: and height: are placed in each image's style. In this example of full image code, the maroon is the link wrapper and the green the displayed image code:
The area of interest is green: the size folder is bold and the style properties that need to be modified are bold and italic
In the example, large was selected during upload: that can be seen in the folder /s400 and width: 400px. Keep in mind the Blogger sizes are long-side, so if the image were higher than wide, the 400px would be the height:.
The size of the displayed image can be increased by changing BOTH the size folder, and removing or replacing the width and height values. Retaining the width and height has the advantage of reserving a space of the image's dimensions. If the image size is important to the layout, even if it doesn't appear, the format will be preserved.
I'll increase the size to 800px wide and keep the width and height properties. Scaling the image to 800px wide is as simple as: (800 * 300)/400=600. So, I'll change the folder from /s400 to /s800 and width: 400px; height 300px; to width: 800px; height: 600px;.
Showing only the green (maroon isn't changed:)
If I elected to remove the width: and height: properties, it would be:
Below are some other sizes I've been testing. The size used to replace "s400" is indicated as size folder beneath the images.
The image display has been reliable for more than 1½ years. Here's my theory: Blogger will deliver multiples of the "official" sizes.
s320 X 2 = s640 s400 X 2 = s800 s320 X 4 = s1,280
Remember, those are long-side sizes.
Also, Blogger seems willing to deliver the Picasaweb sizes in a Blogger address in addition to the Picasaweb address: Thumbnail = s144 = 144px Small = s288 = 288px
Picasaweb also uses: Medium = s400 = 400px Large = s800 = 800px
I added 512px which means 1,024px should display, as well.