I might have to give a 60mm a go - metrification is the word you were looking for :)
Thanks to everyone who chimed in on this thread - sounds like the large rotary saves time and money, since the blade lasts longer and you can cut through more layers. (although with my small amount of experience - I'm only attempting a couple of layers max)
I also find that no matter how hard I press down on the ruler (been using a 12x12" tonight) the cutter wants to push the ruler making me have to line it up again, half way through the cut - grrr frustrating.