diff --git a/src/window_type.h b/src/window_type.h
--- a/src/window_type.h
+++ b/src/window_type.h
@@ -93,6 +93,14 @@ enum WindowClass {
 };
 
 struct Window;
+struct WindowEvent;
 typedef int32 WindowNumber;
 
+/**
+ * You cannot 100% reliably calculate the biggest custom struct as
+ * the number of pointers in it and alignment will have a huge impact.
+ * 96 is the largest window-size for 64-bit machines currently.
+ */
+#define WINDOW_CUSTOM_SIZE 96
+
 #endif /* WINDOW_TYPE_H */