A P2WPKH input should be conservatively estimated with 68.0 vbytes. Standard P2WPKH inputs generally weigh 67.75 or 68.0 vbytes, depending on whether the r-value in the signature is low or high. A wallet using signature grinding will always produce 67.75 vbyte inputs (which is highly recommended). 68.0 vbytes is the conservative estimate allowing for high-r signatures.
Composition of a P2WPKH input
Each input commits to spending a specific UTXO by providing its transaction outpoint:
PREVOUT: hash (32 bytes) index (4 bytes)
The scriptsig for a P2WPKH input is empty, however, the scriptsig length must be provided as
SCRIPTSIG: length (1 byte) <no content>
Each transaction input has its own sequence number:
sequence (4 bytes)
A P2WPKH input requires a witness in the transaction’s witness block:
WITNESS: item count (1 byte) signature length (1 byte) signature (71 or 72 bytes)¹ pubkey length (1 byte) pubkey (33 bytes)
Conservative weight, vsize, and size estimate
A P2WPKH transaction input adds to a transactions…
4 × (32 + 4 + 1 + 4) + 1 + 1 + 72 + 1 + 33 = 272 WU
32 + 4 + 1 + 4 + (1 + 1 + 72 + 1 + 33) / 4 = 68 vbytes
serialized byte length:
32 + 4 + 1 + 4 + 1 + 1 + 72 + 1 + 33 = 149 bytes
If the signing wallet uses signature grinding, the r-value is always 32 bytes, reducing the signature to 71 bytes, and the above maxima to 271 WU, 67.75 vbytes, and 148 bytes respectively.
Note that adding at least one segwit input adds 2 witness bytes to the transaction header, the witness marker and witness flag. Also, when there is at least one segwit input, the witness block must have a witness count item for every input, which must be provided as
0 for non-segwit inputs.
The overall transaction elements of a segwit transaction therefore are:
version (4 bytes) witness marker (1 WU) witness flag (1 WU) input count (1 byte) inputs (variable size) output count (1 byte) outputs (variable size) witness item counts (1 WU per input) witnesses (variable weight) locktime (4 bytes)
Mentioned for completeness, a non-standard signature with both high-s and high-r weighs 273 WU, 68.25 vbytes, and 150 bytes.
¹ also see What is the maximum size of a DER encoded ECDSA signature?.