The Adafruit comments were not wrong but rather confusion about flash size limit being 16 bit words not bytes. So using a USBtiny you can work with avr chips with up to 128KB (64K flash words) or less. That covers most all AVR mega chips except the mega2560 that has 256KB (128K flash words). That make sense?
Arha ! got it!, that makes perfect sense now when you write it out - Yes I was being confused by that. TY.
Checked the signatures are the same on both MCUs 0x000, 0x0002, 0x0004, 0x0001 so that will be ok.
Which bootloader and board description to use is my next quest!
Ta very much for the clarifications - Steve