window dimensions saved in config file should ignore fullscreen mode
There are 6 config options related to window size/placement: ui_last/window_x, ui_last/window_w, ui_last/window_y, ui_last/window_h, ui_last/fullscreen, ui_last/maximized. At runtime GTK (or WM itself, I'm not sure) saves last position and dimensions of dt in windowed mode so you can switch to fullscreen or maximized mode and back without losing your preferred placement and size. But if you exit darktable while in fullscreen or maximized mode, current (big) window size gets written to config and after restart and return to windowed mode you will get some default (and quite small) window size. This of course is just a nuisance, but IMHO it's better to save last windowed dimensions/position in config file and use them as default window size of newly started darktable.