i am getting a segmentation when my array is above 5 and dont know what is cause it

here is my code(coding in C):

#include

int main()

{

int size; /* size of the array */

int grades[size]; /* grades inputed by user */

int count; /* array position count */

double avg; /* average grade of the class */

int fail; /* number of failing grades */

double sum;

size = 0;

/*

* Asks the user the number of grades they would like to input

*/

printf("How many grades would you like to input today? ");

scanf("%d", &size);

/*

* Ask user to input grades

*/

for (count = 0; count < size; count++)

{

printf("Please enter each grade one by one: ");

scanf("%d", &grades[count]);

}

/*

* Finds the average of the grades that the user inputs

*/

for (count = 0; count < size; count++)

{

sum = sum + grades[count];

avg = sum/size;

}

printf("The average grade is %.2lf ", avg);

/*

* Calculates the number of failing grades

*/

for (count = 0; count < size; count++)

{

while (grades[count] < 70)

{

fail++;

break;

}

}

printf("The number of failing grades is %d ", fail);

return 0;

}