In the official BitTorrent protocol specification (BEP 3), piece sizes are flexible, but they are typically determined by the creator of the torrent file.
If you were to create a torrent for a 1TB file using a standard 4MB piece size, the .torrent file would be roughly 32MB in size. That is too large for many trackers to handle and takes too long to parse. By increasing the piece size to 64MB, the .torrent file shrinks down to a manageable size (roughly 2MB). utorrent unsupported piece size 64mb
When your version of uTorrent reads the metadata and sees a piece size of 64MB, it panics. It hits a hard-coded check in its programming that says, "This is too big," and throws the error: In the official BitTorrent protocol specification (BEP 3),