You don't need to use IPropertyStore if you have an IShellItem2 reference, you can directly use IShellItem2::GetUInt64 . Here is some sample code:

CoInitialize(NULL);

...

IShellItem2* item;
if (SUCCEEDED(SHCreateItemFromParsingName(L"c:\\myPath\\myFile.ext", NULL, IID_PPV_ARGS(&item))))
{
  ULONGLONG size;
  if (SUCCEEDED(item->GetUInt64(PKEY_Size, &size))) // include propkey.h
  {
    ... use size ...
  }
  item->Release();
}

...

CoUninitialize();

If you already have an IShellItem reference (in general you want to get an IShellItem2 directly) and want a IShellItem2, you can do this:

IShellItem2* item2;
if (SUCCEEDED(item->QueryInterface(&item2)))
{
    ... use IShellItem2 ...
}

Another way of doing it, w/o using IShellItem2, is this:

IShellItem* item;
if (SUCCEEDED(SHCreateItemFromParsingName(L"c:\\myPath\\myFile.ext", NULL, IID_PPV_ARGS(&item))))
{
  IPropertyStore* ps;
  if (SUCCEEDED(item->BindToHandler(NULL, BHID_PropertyStore, IID_PPV_ARGS(&ps))))
  {
    PROPVARIANT pv;
    PropVariantInit(&pv);
    if (SUCCEEDED(ps->GetValue(PKEY_Size, &pv)))  // include propkey.h
    {
      ULONGLONG size;
      if (SUCCEEDED(PropVariantToUInt64(pv, &size))) // include propvarutil.h
      {
        ... use size ...
      }
      PropVariantClear(&pv);
    }
    ps->Release();
  }
  item->Release();
}