How to read the amount of memory block consumed by pointer?

Question

There is a creation of a global accessible structure as:

struct dummy1 {
int data1;
int data2[10];
} DATA_STORE_A;

struct dummy2 {
int data1;
int data2[10];
int extended_data[20];
} DATA_STORE_B;

struct sample {
int  one_value;
int  value_array[30];
void *dynamic_data;
} SAMPLE_STORE;   // Global accessible structure


SAMPLE_STORE user_init(SAMPLE_STORE sample_instance_one, bool flag, int size) {
    if (flag) {
        
    } else {
        sample_instance_one.dynamic_data = malloc(sizeof(DATA_STORE_B) * size);
    }

    return sample_instance_one;
}

Use case would be to read the correct size of dynamic_data member from structure SAMPLE_STORE

Current approach is:

int main() {
    // Write C code here
    SAMPLE_STORE sample_instance_one;
    bool flag = true;
    uint8_t size = 10;

    printf("Size of sample_instance_one: %lu bytes\n", sizeof(sample_instance_one));
    printf("Size of SAMPLE_STORE: %lu bytes\n", sizeof(SAMPLE_STORE));
    printf("Size of DATA_STORE_A: %lu bytes\n", sizeof(DATA_STORE_A));
    printf("Size of DATA_STORE_B: %lu bytes\n", sizeof(DATA_STORE_B));
    if (flag) {
        sample_instance_one.dynamic_data = malloc(sizeof(DATA_STORE_A) * size);
        printf("Size of DATA_STORE_A(sample_instance_one.dynamic_data): %lu bytes\n", sizeof((DATA_STORE_A*)sample_instance_one.dynamic_data));
    } else {
        sample_instance_one.dynamic_data = malloc(sizeof(DATA_STORE_B) * size);
        printf("Size of DATA_STORE_B(sample_instance_one.dynamic_data): %lu bytes\n", sizeof((DATA_STORE_B*)sample_instance_one.dynamic_data));
    }

    
    return 0;
}

However, in this case the drawbacks are:

1. VOID pointer has to be typecaster to appropriate structure, and for any other module process it may not be known value.
2. Even though being (1) in to consideration at conditionally, another problem is with the sizeof return size is 8 bytes instead of actual size allocated by malloc function.

How can a program retrive actual consumed/allocated size?

sizeof((DATA_STORE_A*)sample_instance_one.dynamic_data) or sizeof(sample_instance_one.dynamic_data) does not return the actual size of allocated memory