Newbe needing help with fixed string length
The printf() family of functions comes handy when you need
this kind of control over the formatted output:

/*
 * Format a number as 5 characters.
 * Returns the string in a statically allocated buffer.
 */
char * format5(int x)
{
    static char buffer[6];  // 5 chars + terminating NUL
    snprintf(buffer, sizeof buffer, "%5d", x);
    return buffer;
}

void setup() {
    Serial.begin(9600);
    Serial.println(format5(15));
    Serial.println(format5(-15));
    Serial.println(format5(-1500));
    Serial.println(format5(1500));
}

void loop(){}


output:

   15
  -15
-1500
 1500