Hi,
As you know that the LPC5410x family has dedicated FIFO system for usart and SPI, the FIFO size is case by case, which are saved in a register called USART FIFO global control register (FIFOCTLUSART), pls read RXFIFOTOTAL/TXFIFOTOTAL bits of FIFOCTLUSART in your application code to know the transmitter/receiver FIFO size.
Hope it can help you
BR
XiangJun Rong