It seems mine are the "40mm" blocks http://www.hobbyking.com/hobbyking/s...mm_5sets_.html
, though I don't know why they call them that. I bought both sizes, and it's the bigger one, but they're about 25mm by 36mm. The measurements on HobbyKings site don't make sense - they seem to have them the wrong way round.
It might be safest to buy one set of each, to make sure
They're cheap enough anyway, and they have other uses. I used two to make a radio controlled launch trolley for a plane with no undercarriage (for the steerable axles). I never got the plane in the air with it, but it was quite fun driving it around.