The limit should be based on the course and site limits (whichever is smallest).
The limits within your PHP.ini file will also come into play (although the site limits should be based on that). Make sure that both the 'post_max_size' and the 'upload_max_filesize' are large enough (note that the post_max_size will need to be slightly larger than the 'upload_max_filesize', as there are a few bytes of additional information included with each file upload).
If that doesn't help, please copy and paste the exact error message here, so I look into why this is not working.