How much RAM should I get for a cloud-hosted PostgreSQL data warehouse?
Estimating database server memory size by the size of tables (or indexes) is going to be misleading. On the one hand, depending on the database workload, it may not ever need to load the entire table (or index) in memory. On the other hand, there are many other possible memory consumers that have little or no correlation to the data size, such as sort buffers or client connections.

If your current server is not memory-bound, then you can use its configuration as a guideline. Otherwise you might as well buy yourself as much memory as you can afford.