HOW many federal employees work for the government? ,About half of them, says the old joke. Fewer than when we got in, says the Clinton administration. Now comes Paul Light of the Brookings Institution, with his book The True Size of Government, _ which examines not simply the size of the federal government but its true scope, shape, and impact.  His conclusion: Governnaent is bigger than we think and, certainly, bigger than what the politicians tell us.