I found a good solution that I want to share with the community.
There are two easy way to set the size of the SPI buffer, one temporary and one permanent:
First of all remove the spidev module from the Kernel:
$ sudo rmmod spidev
then reload it with the required parameter
$ sudo modprobe spidev bufsize=20480
when you reboot the Jetson Nano the value of the size of the SPI buffer will be restored to the default value, i.e. 4096B
To permanently set the value of the size of the SPI buffer you must create a configuration file for
$ sudo nano /etc/modprobe.d/spidev.conf
Enter the following line
options spidev bufsiz=20480
Save and reboot the Jetson Nano
Check the parameter
You can check that the parameter is correcly set using the command
$ cat /sys/module/spidev/parameters/bufsiz
that should output the value of the size of the buffer that you chose, i.e.