How much does SegWit affect transaction size? I know that SegWit transactions are bigger than normal transactions. But how much? 10% or 15% or..?
On bitcoincore.org:


The segwit transaction formats (see BIP 141 - witness program) have the following impact when serialised:

Compared to P2PKH, P2WPKH uses 3 fewer bytes (-1%) in the scriptPubKey, and the same number of witness bytes as P2PKH scriptSig.

Compared to P2SH, P2WSH uses 11 additional bytes (6%) in the scriptPubKey, and the same number of witness bytes as P2SH scriptSig.

Compared to P2PKH, P2WPKH/P2SH uses 21 additional bytes (11%), due to using 24 bytes in scriptPubKey, 3 fewer bytes in scriptSig than in P2PKH scriptPubKey, and the same number of witness bytes as P2PKH scriptSig.

Compared to P2SH, P2WSH/P2SH uses 35 additional bytes (19%), due to using 24 bytes in scriptPubKey, 11 additional bytes in scriptSig compared to P2SH scriptPubKey, and the same number of witness bytes as P2SH scriptSig.



And &quot;How much does it affect block size?&quot;


A witness commitment is included in the coinbase transaction, adding between 38 and 47 bytes, or about 0.005% of a block. (See BIP 141 - commitment structure)

A new transaction serialisation that includes the segregated witness data is defined (see BIP 141, or BIP 144). This adds an overhead of 2 bytes per transaction to allow the serialisation formats to be easily distinguished, and an overhead of 1 byte per input for the count of witness items for each input. These combine to about 1% per transaction.