Once the entity types have been identified, you should estimate the length of each entity. To calculate each entity's length, add up the length of the data elements contained in the entity. Don't forget to include foreign keys residing in that entity. If the entity has a variable length, estimate the maximum possible length of the entity.
Although the lengths of entities may change as you refine the physical design, it is useful to have an estimate of the size of an entity during the design process.
Indicating the length
Once you have determined the length of a particular database entity, you can indicate this information in the data structure diagram. The example below shows the OFFICE entity with a length of 55.
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