What defines the standard Bitcoin transaction size? The Lightning Network specifications limit the number of (unsettled) HTLCs in a channel to 483 at each side (maximum 967 total). The rationale behind this is that the penalty transaction must be under certain size:


  With a maximum standard weight of 400000 bytes, the maximum number of HTLCs that can be swept in a single transaction is as follows ...


What is a "maximum standard weight"? Where does the number 400000 come from?
What is a "maximum standard weight"?


It is a property that nodes follow to prevent processing large transactions with number of rules and signatures. It is standardness not consensus. If a node receives a transaction greater than 400,000 WU, it will discard the transaction and not relay it further. I can however, definitely create a transaction greater than 400,000 WU and contact a miner to mine it in a block. The nodes then will be forced to accept that transaction in the block as it is allowed by the consensus rules. So standardness is a subset of validity that nodes use to prevent large processing and DoS attacks.

Now, if I have to close the channel unilaterally and the channel had a bunch of HTLCs, I can't be expected to go looking for a miner every time my transaction size is greater than 400,000 WU. The other alternative will be to run another software other than Bitcoin-Core that accepts such transactions, but that would have to also mean the miners also run this software. If miners run only Bitcoin-Core they will never receive the transactions and Bitcoin-Core nodes in between will not relay it. The best thing is to conform the Bitcoin-Core standardness as it is in the interest of all.


  Where does the number 400000 come from?


Before the activation of SegWit transactions were limited to 100,000 bytes (since a long time). This was merely extended to 400,000 WU.