diff --git a/src/genworld_gui.cpp b/src/genworld_gui.cpp
--- a/src/genworld_gui.cpp
+++ b/src/genworld_gui.cpp
@@ -48,6 +48,7 @@ struct generate_d {
 	uint y;
 	char name[64];
 };
+assert_compile(WINDOW_CUSTOM_SIZE >= sizeof(generate_d));
 
 extern void SwitchMode(int new_mode);
 
diff --git a/src/newgrf_gui.cpp b/src/newgrf_gui.cpp
--- a/src/newgrf_gui.cpp
+++ b/src/newgrf_gui.cpp
@@ -114,6 +114,7 @@ struct newgrf_add_d {
 	GRFConfig **list;
 	const GRFConfig *sel;
 };
+assert_compile(WINDOW_CUSTOM_SIZE >= sizeof(newgrf_add_d));
 
 /* Names of the add a newgrf window widgets */
 enum AddNewGRFWindowWidgets {
diff --git a/src/signs_gui.cpp b/src/signs_gui.cpp
--- a/src/signs_gui.cpp
+++ b/src/signs_gui.cpp
@@ -163,6 +163,7 @@ void ShowSignList()
 struct editsign_d : querystr_d {
 	SignID cur_sign;
 };
+assert_compile(WINDOW_CUSTOM_SIZE >= sizeof(editsign_d));
 
 enum QueryEditSignWidgets {
 	QUERY_EDIT_SIGN_WIDGET_TEXT = 3,