The best size for a pool depends on the aggregate resources needed for all databases in the pool. This involves determining the following:
Maximum resources utilized by all databases in the pool (either maximum DTUs or maximum vCores depending on your choice of resourcing model).
Maximum storage bytes utilized by all databases in the pool.
Note: Elastic pools enable the developer to purchase resources for a pool shared by multiple databases to accommodate unpredictable periods of usage by individual databases. You can configure resources for the pool based either on the DTU-based purchasing model or the vCore-based purchasing model.
References:
https://docs.microsoft.com/en-us/azure/sql-database/sql-database-elastic-pool