I found that the default is 2 megabyte with post upload limit of 8 megs. This is not enough these days anymore.
I found in Ubuntu 18 with PHP 7.2, that I have my php.ini file to reside here: /etc/php/7.2/apache2
Edit it:
sudo nano -w /etc/php/7.2/apache2/php.ini
And find:
post_max_size = 8M
Change this to a higher value like 16M
Then find:
upload_max_filesize = 2M
Change this to a higher value like 8M or 12M
The values depend on your server hardware, and what you wish to upload to your sites.
Then restart the web server:
sudo service apache2 restart