>In my experience if the thumbs are too small I have to click to see what the image is about

Sure - I call this this "5chan effect". Still the thumbnails for replies are smaller than the parent, and those don't garner complaints for being too small, so there does seem a range there to find that balance.

Another suggestion: for kareha (which I rather like), hide the posting form at the bottom of each thread until requested. It uses up a lot of real estate on a page of many threads.