https://medium.com/@jimmysong/understanding-segwit-block-size-fd901b87c9d4So How Big Can Segwit Blocks Get?
Blocks received by legacy nodes will be less than 1,000,000 bytes. Blocks received by Segwit nodes, on the other hand, can be bigger, but how much bigger?
It turns out that if you made a pathologically large Segwit transaction, you can make a block with just the coinbase transaction and a pathologically large Segwit transaction that’s very close to 4MB. Essentially, this pathologically large Segwit transaction would be mostly Witness Data with the block weight just under 4,000,000. That block would be very close to 4MB, but way under 1,000,000 bytes when stripped of witness data. This is an extraordinary case and wouldn’t be very profitable for a miner unless that transaction also had an extraordinarily high fee.
The normal case without pathologically large/giant fee Segwit transactions results in a block size of around 2MB, which is what the creators of Segwit designed for. When you hear someone say “Segwit is a block size increase”, this is what they’re referring to. The average Segwit block size will be roughly 2MB, though Legacy nodes will still receive blocks that are 1,000,000 bytes or lower due to stripped witness data.
