Is it possible to increase the feed length?

Answer: Unfortunately no, the maximum feed length is currently 50 documents and it cannot be changed. To access document metadata beyond this limit please use the Bookmarks List API instead. However, be aware that usage of the API (as opposed to the feed) is subjected to request limiting. So use some form of client side caching to avoid being throttled.

Is it possible to have a custom sort order of the feed?

Answer: The documents in the feed are sorted according to when they were added to the corresponding stack. The latest document will be at the top of the feed. It is currently not possible to change the order.