Changes over time. Do not optimize for that, because in a year this will no longer be valid. I remember seeing ~350 kB per piece in early 2024, which then slowly halved over the year to ~120 kB.
To get at least some idea about the current value, you could usually estimate the average size by looking at Storj-published stats, and divide the storage_remote_bytes by storage_remote_segments and by the smallest number of Reed-Solomon code parameters (currently 29), but recently stats became funky and report zeros. That would be a network-wide estimate, including pieces of different ages though—not representative of the distribution of pieces that get created now.
And specifically on my long-running nodes—again, might not be representative of the traffic right now—the average number ranges from 261 kB to 304 kB, plus or minus a negligible amount of non-piecestore files.