why height is fixed in standard cell
Well, in a Standard-cell based design, the dimensions of the standard cells are fixed. For example, a standard cell inverter will have a cell width of 12λ and a cell height of 20λ. You cannot exceed these rules. These are designed in such as way to reduce the area and also the delay. Since the area is minimized, the delay also gets minimized due to the lesser metal used for interconnects. So, the area and delay problems are taken care of.
For a particular technology, the power consumption will be known. As you know, in CMOS ckts, the main power consumption is in the Dynamic power consumption. But you also have to take into account the leakage power and other factors.
In general, the standard cell based design is used to reduce the area of designs and also for design reuse. It makes the physical design part of chip design easier and also reduced the time-to-market. Moreover, since the cell libraries are provided by the fabrication houses, when a design is done using these libraries, it takes them lesser time to implement on Silicon and also all the parameters are already known.