You want the size of your resistors to use for your leds.
Let's say you hook your blaster core up to an input voltage of 9 volts from a normal battery.
If we follow Ohm's Law, that gives us the formula for R=V/I or Resistance = Volt / current. The maximum current you may draw through your LEDs per instruction in the PDF is 2.25 mA. Now you know 2 of 3 variables, and can solve the equation. If we add the numbers into the formula we get R = (9 - 3.3)/ 0.0025 = 228 ohm
Es is the source voltage, measured in volts (V), Eled is the voltage drop across the LED, measured in volts (V), Iled is the current through the LED, measured in Amperes (Amps/A), and R is the resistance, measured in Ohms (Ω).
http://www.ohmslawcalculator.com/led_resistor_calculator.php