You should enable it in order to make it the right size with i3wm, but for common window managers, you have to let it disabled.
@@ -35,6 +35,7 @@ config/icon="res://icon.png"
window/size/width=400
window/size/height=400
+window/size/resizable=false
window/size/test_width=400
window/size/test_height=400
window/handheld/orientation="portrait"