Using Clique decision to solve Clique optimization
You would use binary search.  Start with the lower bound being 3 and the upper bound $n$, where $n$ is the number of vertices.  Call your clique decision oracle with a $k$ value halfway between the two bounds.  If it answers "yes", move your lower bound to $k + 1$.  If it answers "no", move your upper bound down to $k - 1$.  Repeat until you have found the largest $k$ value the oracle answers "yes" to.  It should take $O(\log n)$ calls to the oracle.