There is absolutely need to increase it beyond 250. The RSS feed is automatically paginated,which ensures that all products are loaded in subsequent iteration of the feed, in a round robin fashion. This is applicable to blog articles as well. We choose to call it Smart Pagination.
Example: Suppose your store has 1000 products, then the first feed fetch shall display products 1-250, arranged in the descending order of date of creation. The second, third and fourth fetch shall display products 251-500, 501-750 and 751-1000 respectively. There is no upper limit to it. Even if your store has 100,000 products all of them shall be displayed in the RSS feed sequentially.
Why the limit is 250, why not more?
One may ask why we have hardcoded the limit to 250. Actually, this is the maximum number of products that could be fetched via Shopify API in a single request. Our experiments showed that making more than 1 requests to fetch the products via API significantly delayed the RSS feed generation time. So we decided to stick to 250.
The deprecated “page” parameter
Earlier, there used to be a page parameter, used for fetching specific set of products in the RSS feed. This has been deprecated due to underlying technical changes by Shopify. The page parameter no longer works with the app.