A contender for longest may be L’N’H’R, (pronounced “L apostrophe N apostrophe H apostrophe R”).
He played one game in 1912 (Ty Cobb was kicked out for beating up a spectator, and the rest of the Tigers sat out the following game in a show of support for him). The owner scraped together a team of amateurs at the last minute, one of whose name appeared in the scorecard as “L’N’H’R”.
A years-later account of the game speculated, “No one knows to this day if his name was Loopenhower or Leninhasinger, and I bet his wife still calls him a liar when he swears he once played for the Detroits”.
This MLB player had a very short name, which happened to also be the only full player name in major league history that rhymed with the name of his team:
An episode of “This Week In Baseball” circa 1978 addressed this very question. Ed Ott had the shortest name. Then they introduced a new guy with, IIRC, the longest name in baseball (history?) I can’t for the life of me remember what the name was, except that it was foreign (possibly Dutch), and the guy’s career never went anywhere.
UPDATE After a brief Google search, I located a detailed description of the episode in question. It aired August 7, 1979. The gentleman with the longest name was Wim Remmerswaal (Winnefred Nathan Remmerswaal) of Boston. That would have been the week after my youngest brother was born (he died a couple of years ago).
There could be new sheriff in town someday soon, however. Braves’ farmhand Jarrod Saltalamacchia is currently hitting .308 with 12 HRs this season for the Myrtle Beach Pelicans in the high A Carolina League. That’s 14 letters without any steenking spaces – and six more in his first name.