Not sure whether this player is the tallest in pro baseball history, but he must come close. He is 7'1" tall. An article in 2012, when he was a non-roster invitee with the Los Angeles Angels, said he was the tallest player in pro baseball at the time. He also appeared in a big league camp in 2010, as he was on the Minnesota Twins' spring roster that year.
The attached photo is from a website called alchetron.com. The shorter Twins player was not identified. As noted in his baseball-reference entry, Loek is from the Netherlands, and was still an active pro player in 2016.
Loek Van Mil Register Statistics & History | Baseball-Reference.com