With the exception of his funeral effigy, we have a couple comments from chroniclers. We know from examination of his remains that he was between 5'5" and 5'7", on the shorter side of average for his period. Gerald of Wales says that John (and Geoffrey) were both "a little below middle height" but "well-shaped enough". He also describes in the stories about Ireland that John had a fashioned close-cropped beard, as opposed to the long bears of the Irish lords. (Gerald then goes on about how John is a silly little rich boy and I totally recommend reading his descriptions of the sons of Henry II, they are beautiful.) John is also said to have favored in appearance the count of Poitou - so that could be Richard or maybe his grandfather, William IX. Either way, probably means he had red hair and maybe it was curly. I remember reading somewhere and I'd have to grab the source later that suggested that little Henry's droopy eyes came from John, but I don't think I've ever seen any comment on that in passages actually about John written while he was alive. There have been suggestions that Matthew Paris may have actually seen and interacted with John while he was King and that the illustration of Paris might be a truer-to-life illustration of him - here's a link to the National Archives scan of the image (https://www.nationalarchives.gov.uk/education/resources/magna-carta/king-john-by-matthew-paris/) I really like this portrait and I want to believe the suggestion that this is as close as we get to the real John.