The Mac 128K to Color Classic II are all the same size, nothing is smaller in the Mac desktop. A IIGS with a monitor would be bigger; an LC with a 12in monitor would be smaller than the IIGS but bigger than the Mac 128K.
Of the Powerbooks, only a PowerBook Duo series is the smallest, though if you need a floppy drive, it needs to be separate. Only an iBook or Powerbook 12in is the exact same case size and the MacBook Air is the same length and width but 1/2 the height - none of those are 68K. The regular PowerBooks are bigger but most have a built-in Floppy Drive and a larger screen. The Mac Portable is the biggest of the Apple Laptops.
All of this would depend on what you want to demonstrate.
BTW - the PB 190 is a little larger than a PB Duo in length and width but twice as thick.
Of the Powerbooks, only a PowerBook Duo series is the smallest, though if you need a floppy drive, it needs to be separate. Only an iBook or Powerbook 12in is the exact same case size and the MacBook Air is the same length and width but 1/2 the height - none of those are 68K. The regular PowerBooks are bigger but most have a built-in Floppy Drive and a larger screen. The Mac Portable is the biggest of the Apple Laptops.
All of this would depend on what you want to demonstrate.
BTW - the PB 190 is a little larger than a PB Duo in length and width but twice as thick.