Xibo isn’t particularly well suited to a shared hosting environment.
So, all needed changed in php.ini can be found here - How do I upload files bigger than 2MB / How can I increase the upload file size limit?
Assuming you have access to it of course, which is not always the case on shared hosting.
You might want to contact your hosting support about it, I’m quite sure it’s more of a web server issue than Xibo itself.