sizeof?

if you want the constructor to determine the size of the arrays, you can use a class template:

template<uint8_t N>
class MyClass {
  private:
    int* myArray;
    size_t arrayLength;
  public:
    MyClass() {
      arrayLength = N;
      myArray = new int[arrayLength];
    }
    size_t getArrayLength (void) {
      return arrayLength;
    }
};

MyClass<10> myInstance;

void setup() {
  Serial.begin(9600);
  Serial.println(myInstance.getArrayLength());
}

void loop() {
  // put your main code here, to run repeatedly:

}