I found it.
Code was:
var map_size_px := size * _tile_map.cell_size
get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_2D, SceneTree.STRETCH_ASPECT_KEEP, map_size_px)
Code is now:
var map_size_px := Vector2(600,400)
get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_2D, SceneTree.STRETCH_ASPECT_KEEP, map_size_px)
The calc was wrong on the mapsizepx
Now I can define it manually and it works.