In general, the categoric size of databases that are appropriate will be the same or similar in Datomic On-Prem and Datomic Cloud.
I should also take this opportunity to point out that there is no hard limit to the number of datoms in either product, and the characteristics that can make larger databases problematic are often logistical and/or dependent on usage and data “shapes”.
One important difference between Cloud and On-Prem is the treatment of single vs. multiple logical databases. It is strongly recommended that only a single primary database be served by a single On-Prem transactor.
In Datomic Cloud, any node in the primary compute group can act as the primary writer for a given database, meaning that you can run multiple logical databases in a single Datomic Cloud system and can scale up the number of nodes in the primary compute group.