Add-Type -TypeDefinition @"
using System;
using System.Runtime.InteropServices;

public class MI {
    [StructLayout(LayoutKind.Sequential)]
    public struct MOUSEINPUT {
        public int dx;
        public int dy;
        public uint mouseData;
        public uint dwFlags;
        public uint time;
        public IntPtr dwExtraInfo;
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct INPUT {
        public uint type;
        public MOUSEINPUT mi;
    }
}
"@

$type = [MI+INPUT]
Write-Host "Type: $($type.FullName)"
Write-Host "IsValueType: $($type.IsValueType)"

try { 
    $s = [Runtime.InteropServices.Marshal]::SizeOf($type)
    Write-Host "SizeOf(type): $s"
} catch { 
    Write-Host "SizeOf(type) failed: $($_.Exception.Message)"
}

$inst = [Activator]::CreateInstance($type)
try { 
    $s2 = [Runtime.InteropServices.Marshal]::SizeOf($inst)
    Write-Host "SizeOf(inst): $s2"
} catch { 
    Write-Host "SizeOf(inst) failed: $($_.Exception.Message)"
}