Potentially increase share size

[liamsi]

We currently set the share size (rather arbitrarily) to 256 bytes. IMO this should be increased to maybe 1KB or more. Independent of what I think there must be a rationale document for the parameter choice explaining pros/cons and why we settled on whatever value.

[adlerjohn]

To provide some additional context: since

1. Requests with a reserved namespace ID are concatenated contiguously, and
2. Messages are expected to be quite large, as they're supposed to be rollup blocks (i.e. transaction batches),

it makes sense to make the share size larger rather than smaller.

[musalbas]

The main drawback of increasing the chunk size is that it makes data availability sampling more expensive, but we should do some calculations on this.

[rootulp]

Related: #711

[rootulp]

Closed by #850