Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32H7x5_CM7/FDCAN1/FDCAN_TXESC#0x0
FDCAN Tx Buffer Element Size Configuration Register
Tx Buffer Data Field Size:
https://github.com/cmsis-svd/cmsis-svd-data