as per work given to me for now i have to make only charge controller,load is not decided yet
i have to use 12v/40Ah battery that for fix
so how to select the panel wattage range as i want 21VOC of panel
From the battery used, it appears that your load is expected to be around 100W (when the solar panel is off, the battery must take care of the load) and you need solar panel of around 150W capacity.
This is just a rough idea; you need to calculate more accurately.