Most apps that host your podcast will download your entire RSS feed to check for new content. This means there may become a time when your RSS Feed is so large that downloading it eats up enormous amounts of your subscribers data and time.
If you are using Feedburner and your RSS feed stops updating it is likely that your feed has become too large. Feedburner stops updating feeds when the size of your original feed is larger than 512 KB.
So while there is no defined maximum feed size, subscribe to your own podcast and pay attention to how long it takes to download, and how much storage it requires. If your feed is getting too big you can put a cap on it.