I have been trying to change 2 things on my blog:
1) thumbnail size
2) headings of blog previews (font size)
I have managed to change te thumbs slightly, by changing them in Settings > Media. I also used the suggested "Image Gallery Reloaded" plugin. However, there seems to be a limit to what I can extent the thumbs to. As an example, I set the thumbnail image size to 300px x 300px, but the thumbs on the website are only 180px x 180px.
Regarding the headings, I had a good look in your forum and downloaded a couple of plugins, but nothing seems to work. Please let me know where I can change the font size of the headings that preview all blog posts on the homepage.
Looking forward to hearing from you.