In Corona 5 liked the new Corona Sky: many functions, conveniently, Physical Sky is now visible in viewport. But in HDRI mode there's no Viewport Preview Size option for texture. Probably this omission and it would be nice to add this option.
Screenshot 1 - old variant and HDRI appointed on Sky object.
Screenshot 2 - HDRI mode, in viewport this "Cubism" style?... :(