Derby tables and indexes, known as conglomerates, consist of two or more pages. A page is a unit of storage whose size is configurable on a system-wide, database-wide, or conglomerate-specific basis. By default, a conglomerate grows one page at a time until eight pages of user data (or nine pages of total disk use, which includes one page of internal information) have been allocated. (You can configure this behavior; see "derby.storage.initialPages" in Tuning Derby.) After that, it grows eight pages at a time.
The size of a row or column is not limited by the page size. Rows or columns that are longer than the table's page size are automatically wrapped to overflow pages.