<img src="some/path/" class="thumbnail" />
<input type="file" class="image_upload" />
border: 2px solid #ccc;
It is not possible to change the size of a file input. You could redesign the file-input and, but the size of the clickable area isn't modifiable.
Edit: Aaron shows a first trick, and I added the second one, so see this fiddle in which the whole image is clickable for the file input.
The trick is to set
font-size to a large value, then
opacity to zero and finally add
overflow: hidden to the parent element.