How to find out the size of an array of characters?

As the number of responses per student is fixed (in case 5), you can use the function sizeof with only the first element of the vector:

sizeof(respostas[0])

Responding to comment: "the program does not show the typed letters", there is a problem in reading student responses, since the input is storing these responses in a position beyond the vector limit (position 5) in: cin >> respostas[i][5]; .

To solve, just put the index j :

...
for(int j=0; j < 5; j++) {
     cout << "Questão (" << (j+1) << "): ";
     cin >> respostas[i][j]; // nesta linha
};
...

The complete program looks like this:

#include <iostream>
#include <string>
#include <locale.h>
#include <stdio.h>

using std::cout;
using std::cin;
using std::endl;
using std::string;

int main() {
    setlocale(LC_ALL,"Portuguese");

    int n = 0;

    cout << "Quantos alunos?";
    cin >> n;

    char gabarito[5];
    char respostas[n][5];
    string nome[n];

    for(int g = 0; g < 5; g++) {
        cout << "Gabarito: ";
        cin >> gabarito[g];
    };

    for(int i=0; i < n; i++) {
        cout << std::endl;
        cout << "Nome: ";
        cin >> nome[i];

        for(int j=0; j < 5; j++) {
            cout << "Questão (" << (j+1) << "): ";
            // Aqui é j
            cin >> respostas[i][j];
        };
    };

    cout << std::endl;

    cout << "General report"<< std::endl;

    for(int i=0; i < n; i++) {
        cout << nome[i] << ": ";

        // sizeof(respostas[0]) ou sizeof(respostas[i])
        for(int j=0; j < sizeof(respostas[0]); j++)
            cout << respostas[i][j] << " ";
        cout << std::endl;
    };
}

After running, exit the program:

Quantos alunos?3
Gabarito: a
Gabarito: b
Gabarito: c
Gabarito: d
Gabarito: e

Nome: Aluno1
Questão (1): a
Questão (2): a
Questão (3): a
Questão (4): a
Questão (5): a

Nome: Aluno2
Questão (1): b
Questão (2): c
Questão (3): d
Questão (4): a
Questão (5): e

Nome: Aluno3
Questão (1): e
Questão (2): d
Questão (3): c
Questão (4): b
Questão (5): a

General report
Aluno1: a a a a a
Aluno2: b c d a e
Aluno3: e d c b a

Another possibility, perhaps simpler, is to use the std :: vector class of C ++ to store the data.

This class, among other advantages, provides the size() method, which returns the number of vector elements (similarly to length of the Java language.)

Use the sizeof function:

#include <iostream>
...

for (j = 0; j < sizeof(respostas); j++) {
   cout << respostas[i][j];
}