Small pieces on large screens

[tedzards509]

This occurs on my 1440p Desktop screen. It appears to be due to the pieces resolution being too small. I can't test if this also occurs on high resolution phone screens, but if so, it should probably be fixed.

[veloce]

This is probably due the the cacheSize parameter: https://github.com/lichess-org/flutter-chessground/blob/main/lib/src/widgets/piece.dart#L35