Infinity miniatures vary a bit, but tend to be between 28 and 31mm from sole to eyeline. A common comparison is to GW Cadian Imperial Guardsmen which are 28mm sole to eye. Keep in mind also that Infinity miniatures are not as "heroic" as many other ranges. Rather than being bulky and chunky they go the other way with a more animae aesthetic. Most of their soldiers being either fairly realistically proportioned or quite lithe.
Lastly, it bears mentioning that if you put "Infinity Miniatures scale" into google images search, you will get dozens of pictures of infinity miniatures next to other companies miniatures.