Change default tx-size to 350 bytes instead of 200

[vsubhuman]

Original value of 200 byte was taken from the documentation that says: "a transaction of size 200 bytes (a fairly typical size)" (https://cardanodocs.com/cardano/transaction-fees/).

This twitter thread shows the discussion of how 350 bytes seem to be more realistic value that gives more accurate tx-fee value - https://twitter.com/vsubhuman/status/1000039199721099264

Change default tx-size to 350 and change description to point to the calculation.

This is a trivial fix, but it gotta be done properly just so that any change to the formula is transparent and traceable.