I just got an email from the product manager at Yamaha PK here in the US. I had emailed him last night and asked about the size and if a consumer could do the update without having to bring it in to a dealer. Here is what he had to say:
"I'll check on the size with Japan. The OS can be updated through the USB (I've already done this). However, size is an issue for dial-up connecting customers."
George Kaye
Kaye's music Scene
Reseda, California