Recommended pad size for desoldering?
I fail to see in which way having narrower holes may help with desoldering. Less solder to remove? Well, maybe. But it will also make the removal of the part harder as you will have less room for waggling the Arduino pins to break any tiny residual joints in the hole. Thus, slightly wider holes could actually be more useful for the purpose of easy desoldering.

OTOH, bigger pads may help, but only if you use certain desoldering procedures. They can be helpful if you use solder wick, serving as a comfortable landing and working zone for it. In fact, if you're going to use solder wick, leaving some room around the Arduino Micro footprint for the desoldering operation helps too.

Of course, using a socket would be the best way to go. But if you can't due to design constraints, don't be afraid about having to desolder the Arduino. It's all down to using adequate desoldering techniques, it doesn't even require exotic expensive tools. Take a look at this video and see if for yourself.