It is probably not ‘concert hall sized’.
The harp is notably absent from the equipment tables in the Player’s Handbook, so we cannot venture a guess based on its weight.
There is a reference to a particular harp found in Curse of Strahd:
Near the fireplace is a large standing harp.
This tells me that the more detailed description of this harp as large and standing is to differentiate it from the standard expectation of harp size.
Further, in the Sword Coast Adventurer’s Guide, the god of song and poetry, Milil, is described so:
He is variously depicted as young or old, but his identity is always apparent because of his five-stringed harp made of silvery leaves, which he carries constantly.
Depictions of Milil portray him holding an instrument with only five strings, called a harp. A harp can certainly be small in the Dungeons & Dragons canon.
If its size imposed any conditions on its use, it would say so.
Most of the musical instruments listed in the PHB require two hand to play any way. If there were any special restrictions or conditions on the use of the harp, there would be an item description for it that would tell us this. There are no secret rules.
We hand wave the size of things all the time. If I have a carrying capacity of 150 pounds, we just say, “Yeah, Thomas can carry two plate armors on him.”