assuming NOT to use Raspberry’s boot SD card for music storage but just for the system, is there a big advantage in using a Class 10 SD card, or an ordinary class 4 will do?
Also: how big should it be?
Or better asked: how small can it be?
Volumio runs from RAM so the speed of the SD card has no influence to the running system. A slower card just could extend the boot time a bit and will take more time to flash it.