What does universal look like? What is the complexity(size/depth) of universal circuit
The state of the art in optimizing the efficiency of universal circuits is, to my knowledge, this paper. At a high level, a universal circuit to evaluate any size-$s$ circuit has size $O(s\cdot \log s)$ I guess the depth should be roughly similar ($O(s\cdot \log s)$), since a size-$s$ circuit can have depth-$s$ in the worst case, and we do not know of generic techniques to reduce the depth of a circuit while increasing its size by only a logarithmic factor.