To my knowledge none of the leds we use are 20 mm. The MCPCB bases are 20 mm, this may be what you are referring to. The leds upon them range from about 4mm for rebels to about 10mm for lux III, V , P4, Ledengin, etc. There are 10 ( and soon 15 mm) MCPCB's which have rebels on them. Those lenses are specific to each MCPCB and can be acquired at Luxeon Star.
Do you have any photos, specs or more info on the led your asking about?