Ok simply put
.12 or .21 or .15 or anything you see in a car or truck in this range is referring to cubic inches.
a .15 ci engine is approximately 2.5 cc
a .21 ci engine is approximately 3.5 cc
a small block is anything that is physically based off of a .12 ci block, typically all of your 1/10 scale stuff. They make them up to about .18 ci, I think.
A big block is anything based off of a .21 ci block, typically any 1/8 scale stuff. I think they are making big blocks up to .27 inches now.
It is similar to Chevy having the small block and big block, but a little different.
If you are going to get into any serious racing and plan to go to any ROAR events, stay with either a .12 or .21 depending on your racing class. If you are just messing around, get the biggest and most powerful your wallet and truck can handle
Sean