1. Чтение данных из файла

char arrLength(FILE *inputFile, unsigned int *arrSize)
{
	short int currDigit = fgetc(inputFile);
	unsigned char numLen = 0;
	unsigned int currNum = 0;
	while (currDigit != '\n')
	{
		if (47 < currDigit && currDigit < 58)
		{
			currNum = currNum * 10 + (currDigit - 48);
			numLen++;
		}
		else
		{
			return 1;
		}
		currDigit = fgetc(inputFile);
	}
	*arrSize = currNum;
	return 0;
}

int main(int argc, char *argv[])
{
	unsigned int arrSize;
	FILE *inputFile = fopen(argv[1], "r");
	if (arrLength(inputFile, &arrSize))
	{
		printf("Invalid file format!\n");
		return 1;
	}
	return 0;
}

Вариант реорганизации и исправления кода

int main(int argc, char *argv[])
{
	unsigned int arrSize;
	FILE *inputFile = fopen(argv[1], "r");
	if (fscanf(inputFile, "%d\n", &arrSize) != 1)
	{
		printf("Invalid file format!\n");
		return 1;
	}

	return 0;
}