Update/fix block size limits

Bitcoin limits are usually defined using decimal-friendly numbers of bytes. The block size is limited to 1 MB, not 1 MiB, for example. Also, the latest block size limit is 750 KB, not 350 KB.
4 GiB = 4294967296 bytes, which is the correct maximum number of bytes that the block data structure can hold. 4 GB = 4 billion bytes, which is not correct. Similarly, 1 MiB and 1 MB are different, but in this case MB is correct and MiB is incorrect.