This has not been repeated much but months ago it was leaked that the dimensions of the 6.1-inch iPhone 12 are 146.7mm x 71.5mm width x 7.2mm. This is what case makers have been using.
If true, then it is just 2mm taller than the 5.8 inch iPhone 11 Pro and just .1mm wider. Obviously smaller than the 6.1 inch iPhone 11. I assume it has smaller bezels.