mbed-os/drivers/mbed_lib.json

{
    "name": "drivers",
    "config": {
        "uart-serial-txbuf-size": {
            "help": "Default TX buffer size for a UARTSerial instance (unit Bytes))",
            "value": 256
        },
        "uart-serial-rxbuf-size": {
            "help": "Default RX buffer size for a UARTSerial instance (unit Bytes))",
            "value": 256
        }
    }
}