Hello,
The size for custom piece graphics is 64x64 px.
Regarding the sample project, I don't provide free demo files or ZIPs. Since RPG Maker plugins run on open JavaScript, making demo files public makes the source code accessible to anyone on the internet, which creates an obvious risk of piracy. It's just a general policy to protect the code.
If you want to see how the plugin is configured and how it plays, the video on the store page already shows the plugin manager setup and the gameplay.
Also, I regularly update the plugin. A future update is planned to allow using custom PNG images for the board, so you won't be stuck with the current dark wood style and can fully customize it.
Thanks you.