Taco: female cinnamon pearl cockatiel - hatched: ~September 2013, Gotcha date: Nov 29th, 2013
Tiki: Male yellow-sided green cheek conure - hatched April 10th, 2015, Gotcha date May 24, 2015
We have the xs and it is a good size for our green cheek, but a tad to short for our cockatiel because the tiels tail feathers are longer. Mi think a small would be better for a sun.
Green cheeks and cockatiels are smaller than suns so comparing them is no good.
You don't want it too small, you want them to have room to move around in and not have their tail rubbing against everything.
I planned on getting one for Skittles this year but I'm putting it off because I want to get some bloodwork done for him next month (his last was in 2011) at his annual.