I'm not sure what you're specifically looking for, but we do mention support for SQL Server 2008 R2 Express and the 10 GB it can store: ArcGIS Server 10 Functionality Matrix (see page 3, under the Capacity section, Workgroup geodatabase footnote)
The 10 GB storage capacity available for SQL Server 2008 R2 Express databases, also applies to Personal ArcSDE (aka. ArcSDE for SQL Server Express). Please refer to this doc:
Types of geodatabases (scroll down, under the "How is ArcSDE technology included in ArcGIS?" section, 4th paragraph)