This paper addresses various "rules" of planning, but it is important to note that they do not have to be strictly followed 100%. Exceptions will happen, and that’s ok; the realities of life and geography are messy. A general goal would be to follow the rules, say, 90% of the time.
For example: say a neighborhood comprises 20 blocks. If 19 of those blocks are 300 feet by 400 feet (which follow the block size rule) but one is 600 feet by 800 feet (which breaks the rule), the neighborhood as a whole follows the rule sufficiently.
However, if it were the other way around (19 out of the 20 blocks are oversized 600 feet by 800 feet), that’s when you will lose the all benefits of small blocks as outlined in this paper.
There is a grey area between those two examples within which it is safe to operate.