The total size of an index or primary key cannot exceed 900 bytes.
The sum of the lengths of all columns in an index cannot exceed 900 bytes. This error occurs when the size of an individual column exceeds 900 bytes or the sum of several columns exceeds 900 bytes. If you are using multiple columns for an index or primary key, changing the data type of a column to a larger data type can also cause this error.
For more information, see Maximum Size of Index Keys, Creating Indexes (Database Engine) and How to: Modify Column Data Types (Visual Database Tools).