Transistor sizing
u cannot be sizing the transistors for all the three area power and speed in the first case.u need to get ur specs right.then if u want greater speed,then size them more than minimal.be careful cause u can do this only until delay is not dominated by the intrinsic capacitance of the transistor.once intrinsic capacitance starts to dominate,then incereasing size of the transistors will lead to greater delay.now u can see the trade off that when u are increasing transistor sizes,it is leading to an increase in area.now coming to low power, u need to minimise the capacitance at the output node,which can be done by using minimal transistor sizes where ever required.so again u can see a trade off between speed and power.so just keep these things in mind,before anything.
regards
amarnath