these LEDs in your link will work just fine:
http://www.ebay.com/itm/251554148735any colour LEDs should work. The resistors are sized appropirately for all the LEDs I've tried, so you should have no issues.
As far as size, 2*3*4 is great and will work. I have 2*3*4 white LEDs just like those currently in my poker2. Also 3mm will work as long as it doesn't have a flanged base. They're better if you can get flat top, depending on your keycaps.
I recommend a smaller soldering iron tip as the LED pads are rather small.