The Sweet Feet and Beak are kind of annoying because they only fit either vertical or horizontal bars. You can choose which you want, but if your cage has mixed bars or you have multiple cages, it can make it frustrating to not be able to use it more than one location.
Also, I have the mini for my lovebird and linnies and the small (and mini) for my black capped conure. Since conures can vary greatly in size, I'd suggest a small for green cheeks as well but probably the medium for suns and up. I originally bought a medium and gave it to a friend with a Timneh AG.