Depends of what the house looks like, what the rest of windows look like, what the intent is for the room,, what type of windows, etc., etc.

If you're using it for living space, they may need to be wider than 18" for egress. What does the space look like now, and what are you converting it to be?

Match the style, shape, height, etc to the rest of the home. You want to make this look like it’s NOT a garage conversion.

Not enough information. For instance, if converted to any habitable room it will require enough glass and operable opening size to meet light + ventilation calculations. If a sleeping room, a window needs to be emergency escape rated by the manufacturer (so wider) and the bottom placed at a maximum height. If the garage is much lower than the rest of the house, then that may be strange to have really high heads or sills of a window. If you are infilling a former 16' wide garage door within say a larger brick facade, the aesthetic of the size and placement of windows should be addressed. Or, on the other hand, if the conversion went from habitable space to garage space, then none of it matters.