Simple LED resistor calculation:

(Vsupply - Vled) / Iled = R

So for a normal red/yellow/green LED, the forward voltage drop of 1.8V results in:

(6V - 1.8V) / .02 = 210 ohms. Nearest "stock" 5% resistor would be 220 ohms.

For a White or Blue LED with a higher drop, limiting the current more, and using a higher voltage, you would get:

(9V - 3V) / .015 = 400 ohms. You could go with a 390 ohm 5% resistor.

At 5V, 220 ohms will let 20mA through a monochrome LED; 470 limits it to roughly 10mA, and 1K gets it down to approximately 5 mA. Limiting the current saves battery power, but makes the LED proportionally dimmer. If you can remember those 3 resistor/current values, then adjusting upwards for higher voltages is pretty easy.