In C++, you can get the size of a queue using the size() method. Here's an example:

#include <iostream>
#include <queue>

using namespace std;

int main() {
    queue<int> myQueue;
    
    myQueue.push(10);
    myQueue.push(20);
    myQueue.push(30);
    
    cout << "The size of the queue is: " << myQueue.size() << endl;
    
    return 0;
}

In the example above, we first create a queue called myQueue and add three integers to it using the push() method. We then use the size() method to print the size of the queue. The output of this program would be:

The size of the queue is: 3

Note that the size() method returns the number of elements in the queue, not its capacity. The capacity of a queue in C++ is implementation-defined and can vary depending on the system you're running on.