It pushes one item onto the stack, the size of the item on the top of the stack. The item whose size it pushes onto the stack is not consumed, it becomes the second item on the stack.

Here's the source code:

            case OP_SIZE:
            {
                // (in -- in size)
                if (stack.size() < 1)
                    return false;
                CBigNum bn(stacktop(-1).size());
                stack.push_back(bn.getvch());
            }
            break