If you are using Blogger official popular posts widget in your blog, you can see images of that widget are too small (72px × 72px). We can change the size of these images easily. Follow the steps given below:
Go to edit HTML and find this line:
<img alt='' border='0' expr:src='resizeImage(data:post.thumbnail, 72, "1:1")'/>
Change the value 72 to a different value like 120. Now your code will look like this:
<img alt='' border='0' expr:src='resizeImage(data:post.thumbnail, 120, "1:1")'/>
Save your template and refresh your site to see changes.