Now that I created 3D printed shims for the LEDs to nestle under, I was really curious which LED configuration would have the best & brightest light distribution. So I threw together a test board. 

Here's the board. Each one encapsulates both LEDs and resistors under the switch. Starting from left to right:

• Two 0603 LEDs on the outside with their 0603 resistors in the center.
• 0805 LED in the center.
• 0603 LED in the center, resistor perpendicular.
• 0603 LED in the center, lengthwise.
• 0603 LED in the center, widthwise.

Which one do you think was the best?

Drum roll...

• I think the right three were equal in distribution and had a nice glow. 
• The 0805 seemed to be too bright, and centered in one place (not evenly diffused).
• Using 2 LEDs provided excellent distribution of light around the switch, but was brighter than necessary.

For me, the winners (by a small margin), were the right three (single 0603 LED).

Caveat

But, to be honest, it wasn't a fair fight. The brightness of the 0805 LED is spec'd at 150mcd while 0603 is a mere 60mcd. So I'm confident that if you brought the brightness of the 0805 down, it would perform as well as the others.

Technical Details

• Resistors: 200 Ohm
• LEDs
  • 0805: Würth Elektronik 150080RS75000 (red, clear lens, brightness: 150mcd) 
  • 0603: Würth Elektronik 150060SS75000 (red, clear lens, brightness: 60mcd)
• Power: 4.2V, single-cell lithium battery.