The BankStick size will be detected automatically during formatting.
It’s very strange that they have been re-formatted after a firmware update, this shouldn’t happen and may indicate a hardware failure (e.g. short circuit?)
However, you could try following trick: upload the MIDIbox FM firmware, it will re-format the sticks for MBFM usage.
Thereafter upload the MIDIbox SID V2 firmware: BankSticks will be re-formated again - this time for 64k?