Not a comment about the size of the SD card, but I think your bigger concern is that the SD card will hang out of the slot. For whatever reason, starting with the 2010 MBP line, Apple chose to not give us a SD slot that will take all the SD card. A full 1/3 to almost 1/2 of the card remains outside the unit while it is seated in the slot. This isn't a problem when the MBA is stationary, but if you are like me and are constantly slipping the MBA in/out of a sleeve or a backpack all day long, you run the risk of catching it on something and/or even damaging the card.
I've instead started using a 64GB USB 'mini' plug for flash storage. Currently, this holds my entire extended document library on it. That way, no matter which laptop I carry with me for the day, I just slide this into one of the free USB slots and I'm off and running. I use DropBox for my "active" files, but this type of approach gives me something thats very small and unobtrusive and doesn't stick out as far as the SD card would.
Looks like this when plugged in:
I've experimented with putting my entire iTunes library on here but have encountered a few issues using it that way.