The FIFO is 4k in size.
There are only 12 valid endpoint buffer (FIFO) configuration, please look at section 1.17 EZ-USB Endpoint Buffers in the TRM for more details.
8/16 bit width is the configuration of data width you want on the GPIF/Slave FIFO interface. It has nothing to do with the FIFO or the way the data is placed inside the FIFO.
Please read through GPIF and Slave FIFO chapters of the TRM to understand how the interface works. The 16/8 lines are data lines. The interface have other signals and the data lines are used for only data. So you can send or receive more than 255 bytes.
so when a external contoller write a 16 bits wide data on the FD,the fifo buffer is load 2 datum,is it so?
Yep. 16 bits = 2 bytes.