It's just what the name suggests; the size of the block. It's confusing to some, because of ads like the one you mention, but they refer to displacement. You can over bore and stroke some small blocks in order to achieve larger displacement figures, and in fact the factory does that themselves. Take for example the Ford 400M small block, versus the 390 big block. The 400 has more cubic inches, but the 390 is physically bigger. Where this really gets confusing is with Oldsmobiles. Their blocks are all the same size, except for the cylinder height. That is all that separates an Olds big block from an olds small block. The benefit is that most Olds parts, from a 330 to a 455, are interchangable, except for the intake manifold, and on older blocks, the cam & lifters.
Take care,
-Chris
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