Why can’t I upload an audio file that is larger than X MB?
Due to limitations of the API, there is a maximum file size of 200 MB when uploading episodes in WordPress using the Seriously Simple Podcasting plugin.
It's also possible your server or WordPress have limits to the size of the file that can be uploaded as well. These limits can range from 4MB to 128MB. This tutorial includes a step-by-step guide for increasing the maximum file size allowed in WordPress.