I don't have fans for ventilation at the moment and may choose not to have but I have big windows in bathroom. What type of window should i install? The common ones here are those rectangular ones that open from the top, with hinges at the bottom. The window swings open to inside.
The best types of windows for airflow are double hung windows ─ ones that open from the top and bottom. The top part lets out hot air and the bottom part allows fresh, outside air to flow indoors. This works especially well if the top window is near the ceiling, say, within a foot. Windows that open only at the bottom don't really allow a lot of airflow, since they don't allow any air exchange.
If having windows that allow for good ventilation is important to you, then you should invest in one of the following window styles:from http://schmidtroofing.com/best-windows-natural-light-ventilation/
•Awning or Hopper
With a double hung window you can open the bottom and top of the window at the same time. This allows cool air to enter the bottom and hot air to escape out of the top. It makes it possible to ventilate a room using just a single window and to do it effectively. When trying to ventilate a room with single hung windows you need multiple windows in order to create airflow, and they still aren’t as effective as double hung windows at getting rid of warm air.from https://modernize.com/windows/types/double-hung-windows
I'm going to post this as an answer, even though I feel your question is one that cannot have an answer.
A: One that opens
Everything else is subjective. Your environment (wall opening facing N S W E? Location on planet, mountains, valleys, near external water?) plays a big part in this. If you live in a 4 seasons climate, using "just a window" is foolish. Even if it isn't a 4 seasons climate, a cold night, or a hot muggy day, and having max airflow is a BAD THING.
Generally speaking, opening a window of any "normal" size, in a bathroom with a shower for example, is more than sufficient for almost any bathroom unless its a steam room. Of course, the window has to be NEAR the water sources, within reason.