StructLayoutAttribute.Size Field

This field must be equal or greater than the total size, in bytes, of the members of the class or structure. This field is primarily for compiler writers who want to extend the memory occupied by a structure for direct, unmanaged access. For example, you can use this field when working with unions that are not represented in metadata directly.

.NET Framework

.NET Framework Client Profile

Portable Class Library

.NET for Windows Store apps

Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference