Document recommended target block size

[Kleissner]

We should define a recommended target block size. This could be defined as block size that is expected to be always accepted by other peers.

Blocks may be any size; however if they are too big, they won't be requested and stored into the peer's global blockchain cache. There is the config setting CacheMaxBlockSize.

For the decentralized search this is important - for each result it may need to fetch the block.

Pro smaller block size:

• Fast transfer when requested
• Allows caching of more unique blocks

Cons large block size:

• More overhead when there are many files: Requires more disk space, is more inefficient in transfer.

Pro large block size:

• More files per block

Overhead info:

• Each block is signed and the signature takes up 65 bytes.
• The block's total header is 115 bytes.
• A block record (which follows the header) has a basic size of 13 bytes + the payload data.
• A file record uses at least 101 bytes.

Min. block size for storing 1 file (with no additional metadata) = 115 + 13 + 101 = 229 bytes.

[Kleissner]

-> it should be safe to store a file in 1 KB: This includes a description of ~300 bytes + additional 500 bytes available for other metadata.

[Kleissner]

Pictures can be resized to smaller than 4 KB and still can be recognized: https://stackoverflow.com/questions/51353422/compressing-an-image-to-make-it-less-than-4kb