A big block is just that, bigger, in all regard, larger pistons, block, intake, etc.
Basically they allowed more displacement. A general rule of thumb is an engine larger than 400cid is a BB, and smaller is a SB. But that's not really a great guide, since there are SB larger than 400 and BB smaller than 400...
Generally you can just look at it and tell that its bigger. In the case of the ford 351C- I've never heard of that as being referred to as a big block but it does have some gigantic big block style heads. Anyway, when you look at a 302-351 windsor style block and a 429-460 block you would know right away.
displacement doesnt matter. It is the physical size of the engine which determins if it is a big or small block. Example the 352 FE is definately a big block.
Kevin
I believe the manufactureres determined whether or not an engine was a big block or not. Example: Pontiac's 350, 400 and 455 are all based on the same block. It is a large block with a large deck height, very similar is size to a Chevrolet 396 (Big Block). However, they are most often called Wide Block Pontiacs. Never heard the term Big Block Pontiac, but by all rights, it could be called that.
I also tend to see big torque numbers at lower RPMs in Big Blocks but they often don't rev quite as high. But that depends on how they're built.
Fact of the matter being: I don't believe there is a set dimension for big block vs. small block. Check it with your eyes, and ask someone if you're wondering. Pretty much every American OHV V-8 has already been catergorized into one or the other.