If it's a large number of rows you calculate the average rowsize based on datatypes, add the row overhead (14+ bytes) and multiply times the number of rows.
This is the approximate size of the uncompressed table, after applying the various compression techniques it might be anywhere between 10% and 100% of this size.
In the Databse Design manual "Chapter 14: Database-Level Capacity Planning Considerations" there's a lot about sizing.
Dieter
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