Yup, a real bag of worms! As an engine builder, I can tell you, there are as many descriptions of big and small blocks as there are people describibg them.
The Ford blocks are more of a source of confusion than any other of the American V8s. As previously stated, 221, 255, 260, 289, and 302 are "small blocks". So is 351W, with a taller deck and larger mains. But there are those that use the bellhousing pattern as a determining factor with the Fords. That being the case, 351C is also a small block, as it shares the bell housing flange with the others. 351M (Midland) and 400 share the same pattern with Lima (370, 429, 460). So, are they big blocks? Nope. Modified blocks. At least, that's what we call them in the "industry". FE is the "original" big block Ford, with the 390 being the banner carrier (they made about 8 390s for each of the other CID). There was a 330, 332, 352, 360, 361, 383, 390, 391, 406, 410, 427 and 428. (man, what a list, and I probably missed a couple!) Bullit proof engines. Very inefficient when it comes to fuel vs. power.
The Pontiac is perhaps the biggest anomoly of all, having CID anywhere from 287 to 455, all with the same basic "block". As noted earlier, there are variations in bearing diameter and bore diameter, but all the main parameters are the same. Please, don't use "301" and "Pontiac" in the same sentence... And don't forget the 265!
Suffice it to say, an engine "family" shares bore centers, cam-to-crank centers, cylinder head patterns, etc.
One last thing. You will note the Fords are all named after cities. Those cities are where the foundries are that produced that particular engine family. All but Windsor are in Ohio, and Windsor is in Ontario, Canada.