Well, your question does not really have a correct answer. The game's "official" full body images for uniques are 500x1080, as EpitaphGamer stated. So one can infer that the game is designed to display with that ratio in mind (or that future changes will continue to have this ratio as the design base). But, there are multiple places where full body is displayed and the display area available to them are not the same sizes nor ratios. So there is no actual, "just right" size. And while the full body images are 500x1080, the shadow-images used as default in gear selection are 375x430 (which is obviously a vastly different ratio, but still a place where full body is shown).
All that said, what I imagine you are actually asking is, what is the size of the component into which the full body image is shown on the mansion screen that hovers over the master-list of slaves. That thing is currently 370x608.