Sites hosted on Nexcess will already have very high PHP defaults without having to override those values via a .user.ini file.
The following common PHP values are set as follows:
max_file_uploads 1.024 GB
memory_limit 2.048 GB
post_max_size 1.536 GB
upload_max_filesize 1.536 GB
Those commonly requested values on your site for plugins & themes do not need to be changed. This is because the default values for PHP memory and post max and upload max file sizes are already more than high enough.
Some plugins and themes might recommend setting the PHP memory to at least 256 MB in size but the PHP memory by default is already going to be higher. The same is true for upload file size limits that are already high enough in most cases.
Take the time to reference the default PHP values already being set before making any custom PHP default changes using the .user.ini file.