In the config screen you can adjust the size of thumbnail that s9y creates, however I believe this is only based on width, so you would need to make sure your original has a 4:3 aspect ratio to get 200x150.
Dave raises a good point that I had forgotten. Basically, if you insert an image without a comment than s9y adds a style="" into the HTML for the image, and you may need to remove this before you save your entry.
carl_galloway wrote:.....adds a style="" into the HTML for the image, and you may need to remove this before you save your entry.
or use border: 1px solid #color!important to override the inline style, especially if a lot of entries already exist and editing all of them might be a lot of work.