Re:
That'll be for a Shire, they usually grow to around 17 hands, but the biggest has gone to 18.
A hand is 4 inches and the horse is measured to the withers, the point just in front of where a saddle would be, so 17 hands is around 5 1/2 feet to the point where the neck meets the back.
Could be 1950's, by that time heavy horses were rapidly becoming obsolete and there are stories of farms in Suffolk and Norfolk slaughtering 200 or more horses at a time, as they were effectively worthless.
I've ridden a 17.2hh Shire, great fun at speed, but not the brightest things on four legs.