yes as i have no response i did my own experience
i saw in JADESC.asm (descriptor header file) this line
#define EP0_BUFF_SIZE 8 ; 8, 16, 32, or 64
so my conclusion is that i can make a 64 bytes buffer per transfer
i have not yet compiled such a program but i'll do it soon
for the moment i see transfer speed with this HID/full speed example, playing with polling interval