Split large partitions, with more than 20 million rows or that are over 250 MB in size, into smaller partitions to improve performance
This rule analyzes partitions to determine whether existing partitions are too large. A measure group that contains only a single partition is considered too large if it has either an estimated count of more than 20 million rows or an estimated size of more than 250 MB.
For more information about designing aggregations, see Designing Partition Storage and Aggregations in SQL Server Books Online.
For more information about performance related to partitioning, see the section, "Designing partitions," in the SQL Server 2005 Analysis Services Performance Guide.