It is a micro SIM.
I just upgraded to from a first gen Galaxy Tab 10.1 to a Nexus 7. When the N7 arrived, I realized that my standard SIM didn't fit.
I was able to (VERY CAREFULLY) cut down my regular SIM card to the micro SIM size/shape.
I'm still baffled as to why the industry thought we needed smaller SIM cards. I don't think the size of the SIM card is holding back our devices, given that the devices seem to keep getting bigger these days anyway!
-SF