If you are using the official popular posts widget from Blogger, you can see thumbnails of popular posts are too small. Default size of popular posts thumbnails is 72px × 72px. We can change this size easily editing a single line of popular posts widget code. Follow the steps given below:

Go to edit HTML of your blog and find this code:

resizeImage(data:post.featuredImage, 72, "1:1")

Change the value 72 to a different value like 120. Now your code should look like this:

resizeImage(data:post.featuredImage, 120, "1:1")

Now you will get 120px square images as popular posts thumbnails.

If you don't like square images, you can change the 1:1 ratio of width to height. For example if I change the ratio to 3:2 like below, I will get 120px width and 80px height images as popular post thumbnails.

resizeImage(data:post.featuredImage, 120, "3:2")

After that save your template and refresh your site to see changes.