Shoutbox

lParam
    The low-order word of lParam specifies the new width of the client area.
    The high-order word of lParam specifies the new height of the client area.

So,
width = lParam & 0xffff;
height = (lParam >> 16) & 0xffff; // or you can use lParam >>> 16;

lParam is dword (double word), of length of 4-byte. A word, in x86 terms, is 2-byte. And little-endian.