Depending on how you have it configured, you would either find it as an option or a permission.
First check under Admin CP > Applications > Gallery Options > Video Options.
If you have the "Enable Global Quotas" option checked, you can change the max file size there.
If that is unchecked, then you will need to check the permission: Admin CP > Users > User Group Permissions > (Registered) > XenForo Media Gallery Video Media Quotas > Maximum File Size. You may need to apply it to a different user group.
There are similar options/permissions for image uploads too.
If you are looking to accept files of that size, also make sure your server has a high enough post_max_size value. There may be other settings required to handle large uploads.