Ok so to order categories by name you do this;
In the database table, the fields are called cat_name and cat_id. so wp_list_cats add a prefix of 'cat_' to whatever you pass in to this function.
So how can you order them by the number of posts?
Passing in 'category_count' brings up an unknown error as column 'cat_category_count' is missing. Just using 'count' obviously doesn't work either.