[DrDoom]

According to a historical marker at the Union Terminal in Cincinnati, which claims to be near where the grandstands and home plate were, the ballpark was 550ft square. This is unlikely as that is the total distance between Kenner and Hopkins. The only way that would have been possible would be if the Union Grounds actually occupied the entire shaded area of Lincoln Park in the map above. However, shaded areas on that specific map denote public parks which the Union Grounds was not. Another thing of note, in order for the marker to have been anywhere near the grandstands, Union Grounds would have had to have been west of Dalton, between Dalton and McLean - this is consistent with the BR Bullpen entry on the park as opposed to many other sources stating it was located in the western third of Lincoln Park. What is interesting about that is, at 400-ish feet squared it fits neatly between the two streets. Also, McClean was actually a player on the Reds and even was a partial owner of the American Association Reds and Outlaw Reds that would come years later. It would have left some empty space - likely on the south side of the park - but that could've been for carriage parking.
I've attached an image showing what I mean.