I have both the 11" MBA and a Surface Pro 2.
Biggest issue with Surface Pro 2 is not necessarily screen size exactly, but GUI SCALING.
On a Mac we have Hi-DPI mode meaning user interface elements scale well, on the surface pro 2 the setup is different.
Desktop mode for example at default is set to 150% scaling. This makes gui elements easier to navigate on the smaller screen and more over with 'touch'...
For better effect and one that looks really great you can up this to 200% making windows easier to navigate and buttons easier to touch.
In the windows elements part of the equation all is well, GUI scales nicely and everything remains sharp, clear and icons are nice and clean.
BUT..... Then we get to programmes.
Photoshop CC (latest version of photoshop for creative cloud users) doesn't scale its GUI like HI-DPI version on mac. So you end up with tiny menu elements and such.
Worse still other applications scale but badly, resulting in things like onscreen menu bars being cut off, pixelated menu items, and the whole interface is just unusable in that mode.
Corel Painter X3, Sketchbook Pro for example are culprits of this, which means you can not run them properly with GUI scaling set to 150 or 200% and will need to change the whole desktop GUI scaling down to 100 or 125% which means once again you will end up with VERY VERY SMALL menu items and icons which are just awkward and cumbersome to navigate, even with pen input.
So yeah, that's the biggest issue you will have with desktop apps on the Surface Pro 2 until companies actually sort out HiDPI support for their applications on Windows.