次のようなことを試してください:
// find the longest item
CString longest;
for (int i = 0; i < m_list.GetCount(); ++i)
{
CString temp;
m_list.GetText(i, temp);
if (temp.GetLength() > longest.GetLength())
longest = temp;
}
// get the with of the longest item
CSize size = GetWindowDC()->GetTextExtent(longest);
// you need this to keep the current height
RECT rect;
m_list.GetWindowRect(&rect);
// change only width
int width = size.cx;
int height = rect.bottom - rect.top;
m_list.SetWindowPos(NULL, 0, 0, width, height, SWP_NOZORDER | SWP_NOMOVE);