How large a file members can upload is limited in two places:
Jamroom takes the smallest of:
upload_max_filesizepost_max_sizememory_limitFor example, with upload_max_filesize = 64M, post_max_size = 64M and memory_limit = 512M, the maximum is 64 MB. To allow 512 MB uploads you need upload_max_filesize and post_max_size of at least 512M, and memory_limit of at least 1024M.
The System Check shows the current maximum as Max Upload.
These are PHP settings (php.ini, or your hosting control panel's PHP options). On most shared hosting you ask your host to change them. After changing php.ini, restart the web server or PHP-FPM, then open the System Check to confirm the new Max Upload.
Then raise Max Upload Size for the quotas that should get the larger limit.
On Nginx, client_max_body_size must also be at least as large as post_max_size. Otherwise large uploads fail with error 413.