Choosing an SMD LED size
The brightness is not defined by the footprint, but by both what is inside the LED and the lens style.

The data sheet will provide a number of milli-candelas (mcd) for different currents.  The data sheet will also provide a viewing profile - a map of how the light is emitted from the LED.

For working with a light pipe you want to be looking for an LED that has as tight a viewing profile as possible so the maximum amount of light is fed up the pipe and not spread around the interior of your unit.