You must have an incredible server deal. 300 mb is a huge file and I wonder what will happen when 50 people start uploading files that size, or people start watching the videos.
PHP.ini has a default upload limit of 2mb. To increase the limit, you need to find PHP.ini and make the change there. On a VPS server I'm pretty sure that you will have to restart it for the change to take affect. On a shared server you may NEVER convince your host to increase the upload limit to that amount. If you can, he will have to make the change for you.
Keep in mind that the limit covers everything. Photos, stories, files etc. As such I'd be thinking twice about making the change to such a high figure. Most people with VPS servers increase the size to something more reasonable. I'm guessing 50 -100 mb would be the maximum. In my case it's 5mb because I don't allow video uploads. I ask my members to upload the videos to youtube and embed the link.