Window Air conditioners are designed to fit in most size windows.
The larger windows however will need to be retrofitted with insulation or boards to seal the window air conditioner in the window.
Also if you have a sliding window that slides sideways instead of up and down then you will need a window air conditioner that fits those windows or make a board to fit in between the window to seal around the window air conditioner.
Window Air Conditioners are cheaper to run if you're cooling just one or two rooms or cooling a small house.
However if you're trying to use more than 1 window air conditioner to cool a larger house then it's cheaper to use the central air conditioner and it's more efficient as well.
Window air conditioners don't use as much electricity if you're running 1 or 2 of them and if you have a small 5000 BTU air conditioner then it uses around 500 watts of electricity and costs around $20.00 to $30.00 per month to run the window air conditioner.
With a central air conditioner it can cost as much as $300.00 per month in electric cost to run the central air conditioner and the more it has to run the more money it will cost.
My central air conditioner was costing me $300.00 to run and so I would start just cooling a few rooms in the house with my window air conditioners and my bill dropped down to $60.00 per month for 2 window air conditioners running.
I only run the central air when I need to cool the entire house such as when family are over.