What is the tallest horse you have been around?
Mine would be a thoroughbred named Lurch. He was abandoned at a facility and had all sorts of health problems, including a large ball on his elbow that he was due for surgery for before he died. Lurch measured 18.1 at the withers and I do believe that they were trying to get him into Guinness as tallest TB. I'm unsure of whether he had a lip tattoo but I think his lack of papers might have kept him from the title.
Here is Lurch (RIP) about a year before he passed. He was 18.1 Hands. The girl ontop of him is somewhere around 5'10'', I believe. He was ridden in light dressage to keep any kind of muscle on the poor guy.