Consider the following two commands both creating a dumb 1KB file
dd if=/dev/urandom of=test.file bs=1024 count=1 dd if=/dev/urandom of=test.file bs=1 count=1024
The first command uses a block size of 1024 bytes and a block count of 1, the second does the other way round.
My guess is that there is no difference and limiting block size is a RAM related issue: you can't have a block size larger than available memory.
Are there any special cases when I would want or have to use the first case over the second one? And vise versa?