Finding right block-size with JSON loading difficult for user
This was actually a second error arising from a too small block-size in the json reader.

Finding the right block size is difficult for the layman user
As reported by @thomwolf, while loading a JSON Lines file with "json" loading script, he gets
> json.decoder.JSONDecodeError: Extra data: line 2 column 1 (char 383)

Finding right block-size with JSON loading difficult for user 
 As reported by @thomwolf, while loading a JSON Lines file with "json" loading script, he gets
> json.decoder.JSONDecodeError: Extra data: line 2 column 1 (char 383)
 
 This was actually a second error arising from a too small block-size in the json reader.

Finding the right block size is difficult for the layman user