Currently there's no way to change the image size in the EPUB version of a book.
In terms of EPUB, since the images are going to be read on the small screens of phones, this is our way of making things look reasonable in a simple way.
Now, for our web EPUB viewer, we should make some enhancement about being able to click on the image and have, say, a lightboxed full-size version of it.
However, for the time being, this is also accomplished by a right-click and opening the image in new tab, etc.