Rhys Van Nek is listed at 5'10"/177cm, maybe an inch shorter than George Francis (5'11"/180) and a centimetre shorter than Tupou(178cm). Other international props around that height or shorter include Ox Nche (~176cm), Javan Sebastian (~177cm), Beka Gigashvili (~177cm) and Giorgi Melikidze (~178cm). Height/weight listing has always been a bit variable, but I'd guarantee you that inch difference between Francis and Van Nek is accurate given they were packing down in the same front row at Easts alongside Richie Asiata (5'11"/180cm) a couple of years ago and did not look lopsided when they were folding their opposition.
Either way I definitely wouldn't say he's too short as prop, particularly not in set piece - the issue a lot of modern props are seeing is that they're too tall and it can allow their opposition to get underneath them and pop them up in the scrum.