They are sold by size, usually they use 0603 on keyboards but looking at that pic they look smaller than the switch pin holes, so I'd guess they're probably 0402 - I would wait for someone to confirm that though as I'm no expert.
The other important spec is the resistance which would depend on the colour and model of LED you're using and how it's wired up (which as someone else made it we have no idea) For testing purposes assume they're on the 5v from USB and use the specs of whatever LEDs you have in
this calculator - if you get it wrong they'll be too dull so be sure to test one before filling up the whole board!