The Sony Xperia XZ1 Compact was the perfect size phone and I hate the world for not having successors of the same size.

The phone I am currently using has the following dimensions: 165.2 x 75.7 x 9.1 mm (6.50 x 2.98 x 0.36 in). I think it is a sweet spot for me. I would not go for larger than this.