GrabzIt's API and Online Screenshot Tool automatically converts screenshots into website thumbnails by resizing the images until it fits within the maximum size allowed on the user's package.
However if you want to specify your own dimensions for the thumbnail, then the thumbnails width and height should be smaller than the browsers width and height, which is 1024px and 768px respectively by default.
Next it is important that both the width and height dimensions are kept to the same ratio as the browserHeight and browserWidth parameters. For instance if you want the thumbnail to be 50% the size of the browserHeight and browserWidth parameters you would make the width and height parameters 512px and 384px respectively. The GrabzIt API or Online Screenshot Tool will now thumbnail the website to a 512px by 384px image.
To calculate thumbnail dimensions easily and view code examples check out this article on website thumbnails.