Ideal Wordpress memory limit on 16 GB RAM

Hello and thanks for choosing Lightsail!

I did some reading on the memory_limit php.ini option. As indicated by the documentation, the memory limit is applied on a per-script basis. I think that in WordPress that would also imply per-page-request because each request for a page results in a script execution. With that in mind, my advice would be to increase this value only if you know that the plugins you are using might need more than the 512MB limit set by default. I do not think that the size of the instance's memory would or should affect this decision.

I hope this helps!