Alright, here are the results. Make sure to view the attached screenshots at native resolution so you can see the results properly :
I've used the 128 icon for the house, the 256 icon for the storehouse and the 512 icon for the farmstead so I could compare them side by side.
When viewing the buildings in the build in the production queue, the 128 chicken looks best, the others show aliasing (512 has the most aliasing)
However, when looking at the portrait in the information panel (as shown below) it looks like the 128 is slightly blurred (when compared side-by-side, on its own it looks pretty decent). The 512 on the other hand still looks a bit aliased. 256 gives the best result.
So clearly, higher resolution does not give necessarily better results. Instead, one should search for the size where the least resizing is needed to get the best looks.
So, what resolution is better? 128x128 or 256x256? Actually, I think that 128 looks slightly better. I do however think that 256 is more future-proof (higher resolution screens get more and more common so I think I wouldn't be surprised if the GUI gets scaled up (to prevent from getting too small) in the nearby future for HDPI screens.
What resolution looks best according to you, guys?