Lets first quote:
Combined size of all of the properties in an entity cannot exceed 1MB. (for a ROW/Entity) from msdn
My Q
Azure table row size calculation is quite involved and includes both the size of the property name and its value plus some overhead.
http://blogs.msdn.com/b/avkashchauhan/archive/2011/11/30/how-the-size-of-an-entity-is-caclulated-in-windows-azure-table-storage.aspx
Edit. Removed statement that earlier said that size calculation was slightly inaccurate. It is quite accurate.