--------Warning! This post contains links to a website that is NSFW!------
OK, my blog is http://www.roughstraightmen.com
You can see those thumbnails in the right sidebar, somewhere in the middle of the page (you have to scroll down), and the title is "Top Posts & Pages".
As you can see, the thumbs look too small and are barely distinguishable. So I wanted to make these thumbs a bit larger.
The code that you proposed in the first post has been added to my theme's css file, but it seems to have no effect.
By the way, while we are at it, the widget "Popular Posts Tabbed Widget" also doesn't work properly. It displays the default "Wordpress" thumbnail instead of thumbs taken from respective posts. You can see that widget right below the "Top Posts and Pages" widget.
Any idea how to make the Popular Posts Tabbed widget work as well?