How small do CAN transceivers get?
Some MCU can have integrated CAN transceiver
>https://www.nxp.com/products/LPC11C00
If the I2C open-collector bus works well enough, the same bus will work fine for CAN. There is no need to comply with the CAN spec.
Some MCU can have integrated CAN transceiver
>https://www.nxp.com/products/LPC11C00