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In .NET versions prior to .NET 4.5, the largest allocation for any single object is 2GB.

On 64-bit platforms, in .NET versions 4.5 and greater, it is possible to enable the allocation of arrays that are larger than 2 GB in total size (but NOTE this does not change other limits on object size or array size):

• The maximum number of elements in an array is UInt32MaxValue.

• The maximum index in any single dimension is 2,147,483,591 (0x7FFFFFC7) for byte arrays and arrays of single-byte structures, and 2,146,435,071 (0X7FEFFFFF) for other types.

• The maximum size for strings and other non-array objects is unchanged.

The default setting is not enabled.

You can enable this feature by using the gcAllowVeryLargeObjects element in your application configuration file:

<configuration>
  <runtime>
    <gcAllowVeryLargeObjects enabled="true" />
  </runtime>
</configuration>

Before enabling this feature, ensure that your application does not include unsafe code that assumes that all arrays are smaller than 2 GB in size. For example, unsafe code that uses arrays as buffers might be susceptible to buffer overruns if it is written on the assumption that arrays will not exceed 2 GB.

Ref.