The following info was derived from multiple websites discussing the pairwise transient key (PTK). The PTK is a total of 512 bits but is divided into 5 different keys. One of these keys, the TK (temporal key) is 128 bits and that is what is used to encrypt the data frames.
The PTK is formed from PMK + Anonce + Snonce + Client MAC + AP MAC
The PTK is a total of 512 bits and actually contains the following 5 keys
KCK - 128-bit Key Confirmation Key used to provide data integrity during 4 -Way Handshake & Group Key Handshake.
KEK – 128-bit Key Encryption Key used by EAPOL-Key frames to provide data privacy during 4-Way Handshake & Group Key Handshake.
Temporal Key – 128-bit key used to encrypt & decrypt MSDU of 802.11 data frames between user device & access point (confidentiality of data)
Temporal MIC Authenticator Tx – 64-bit key used to compute message integrity key (MIC) - protects integrity of data frames transmitted by the AP to the user device
Temporal MIC Authenticator Rx – 64-bit key used to compute message integrity key (MIC) - protects integrity of data frames transmitted by the user device to the AP