Best practice has long been to keep the heap size below 32GB to benefit from compressed pointers and assign 50% of the available memory available to Elasticsearch to the heap. Based on this best practice the maximum node size is 64GB RAM. Elastic Cloud, which the blog post refers to, follows best practices and there I believe the max node size is indeed 64GB. This is a convention followed by Elastic Cloud and not a technical limitation. If you host your own cluster you can have nodes with more RAM if you want to.
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