Use the whey powder to help you get enough protein in your diet for your goals. Nothing more, nothing less. You need a certain amount of protein to maintain your current biological function and repair muscles. If you want to get bigger you need more protein than the maintainance amount.
As for being bigger or faster, you'll need both. It's relative as to which you should be working on. If you have slug like speed but can toss Volkswagons around, you may want to spend some time on speeding up proper execution of technique. On the other side, being quick doesn't get you too far if you can't move a heavy bag with a good cross or arm bar your sisters teddy bear.