pudzer...
the ammended fire regulations include provision for 'restrictors' to be incorporated onto first floor windows... these work very well IMO.... you may have seen them, they are small metal catches at the bottom of the window, they restrict the openings to usually 100mm.... and can be undone without a key... children cannot undo them.... they are a great idea....
anyway, to answer your question, the minimums and maximum are set by the fire and ventilation regs.... the bottom of the openable section os a window in a bedroom should not be below 800mm (or 600mm in caseof a dormer or rooflight)....... therefore the usual head height is 2.1 m, so a height of 1350mm suits the ope (the window board and window frame brings the opening to over 800mm above floor level)
the fire regs state that a min clear opening of 0.33 sq m min is rquired (with a min dimension of 450mm.... therefore if your window opening is, say 500mm wide then you need at least 660mm height....)
but the ventilation regs come into this as well....
i cannot find it exactly right now, but i think there is some provision for the min height of the head of the window..