Hi Dias, thanks for your message!
There is no limit concerning the size of the USB flashdrive you use.
However, keep in mind that the size and number of soundpacks you load on it will impact the loading time of these files : you can add as much soundpacks as you want but the more you put, the more time it will take for your soundboard to process them.
Don't hesitate if you have any question. Have a nice day!
Marylou