Resistor value for LCD

Can any of you tell me how I would figure out what value resistor is needed for this:

eBay LCD

I could just start trying a bunch but if there is a way to find out, I might as well try and do it right for a change.

Thanks.

Hi! It´s not the same seller but I also bought a 2-LCD Set from China. They are nice and work well with the 68 ohm resistor but they are too flat and they don´t really fit into the cutout of the metal case e. g. Means, they don´t come up to the case level vertically and horizontally they are close to the front but there is a space at the backside. I don´t know how to explain better. It looks ugly.

Same topic: I bought some other 16x2 LCD from china. These fit well into the metal case. I think they have a build-in resistor for the LEDs. When I connect them to +5V directly I measure 1,94V at the LEDs.

The seller recommended 220ohm, which surprised me a bit but seems to work fine.