There are no size limitations from the app.
Image preview uses GD library to generate thumbnails, so any limitations there are, comes from that library (perhaps this can be changed by some configuration, not sure).
Unfortunately there are no other previewers available, partially because haven't got any requests what previewer would be useful. After all, preview should be faster and lighter, and should be able to display the content in a small area, so not all content can be previewed.