if you dont want to open the light and measure yet, just buy both sizes 
they are cheap
you could use the wrong one to practice reflowing ;-)… if youre going to all the trouble of getting a light open and unsoldering the mcpcb, it really helps to be able to reflow onto the stock mcpcb…
no harm buying LEDs premounted, if the mcpcb does not match the light, it is easy to reflow the LED off the mcpcb and move it onto the stock board… you can buy the hotplate I linked for less than $15, so reflowing will not be an obstacle…
Reflowing is actually really Fun!