Yo nunca he necesitado de tal cosa, pero checa un tipo de variable llamada BigInteger, creo que eso es lo que buscas.
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es C#
msdn.microsoft.com/es-es/library/s1ax56ch(v=vs.80).aspx
¿Cómo puedo almacenar números de hasta 1,000 dígitos?
WTF? xD
Quieres decir que tu número máximo puede podría ser: "9999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999"
Es una barbaridad.
No creo que haya un tipo de dato numérico para almacenar tal cantidad.
Estás seguro de lo que pides?