Blog Entry list not making a thumbnail

Hi, I’ve been fiddling with this for sometime but not working it out. I’ve selected thumbnail for the featured image in the blog entry list but it’s big, and if I change the vales for the custom image height/width all it does is ruin the quality but the image stays the same size. I could make a thumbnail size image and use that but that would be annoying. I have changed some CSS:

.blog-entry.post .thumbnail img
{
  width: 50%;
}

Which has made the image more sensible but then the container is still large. I’ve not been able to nail down the container size.
I’d rather the image was a proper thumbnail and the content wider.

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