To increase the upload limit in PHP, add the following lines to the pool:
php_admin_value[post_max_size] = 64M
php_admin_value[upload_max_filesize] = 64M
You also need to add the following line to the corresponding server block in the nginx configuration file:
client_max_body_size 64M;
It is also possible to set client_max_body_size in /etc/nginx/nginx.conf. All sites will then use the specified limit.