
You might find some CSS tricks from the internet, which can resize the thumbnail. Changing the width / height of an image by CSS can only make the image looks larger, but the real size of the image doesn't change. Therefore, the image will look unclear(maybe look like mosaic).
I made this tool which can resize the Blogger popular post thumbnail in real size, and it also includes some other features as below. If you would like to install this tool directly, please jump to "II. Preparation".
(pic from: magicmockups.com)