AlertDialog dialog = builder.setTitle("訊息列表")  
                    .setView(layout)  
                    .create();  
dialog.show();  
//設定視窗的大小  
dialog.getWindow().setLayout(300, 200);  

dialog.show();一定要放在dialog.getWindow().setLayout(300, 200);的前面，否則不起作用。

WindowManager.LayoutParams params = dialog.getWindow().getAttributes();  
params.width = 300;  
params.height = 200;  
dialog.getWindow().setAttributes(params);  
 

final WindowManager.LayoutParams attrs = getAttributes();  
attrs.width = width;  
attrs.height = height;  
if (mCallback != null) {  
    mCallback.onWindowAttributesChanged(attrs);  
}