The shadow Cabinet currently has 34 members in it. In government, this number will have to drop by at least ten. But still, a 24 person group is, probably, too large to foster constructive and detailed debate.
However, if the Cabinet was smaller, then it would be easier to restore it to its traditional role without sacrificing effectiveness. One Cameron aide told me recentlty that 15 was probably the ideal size for the Cabinet and would make it possible to bring back Cabinet government.
So, why doesn't Cameron just slim down the Cabinet? There are two reasons why he won't. First, it would involve a time consuming reorganisation of Whitehall. Second, it would create a lot of ticked off members of the shadow Cabinet. But in the long term a Cabinet of 15, which would have to be roughly represenative of the parliamentary party as whole, would be a better recipe for party unity than a policy board made up almost entirely of Cameron's long marchers.