Affinity Photo 1.8.6 (currently the latest) on macOS 10.13.6.
The selection size displayed in the info panel does not match the actual size of the selection rectangle. Half the time it's smaller by one pixel. This goes for both the width and the height as can be seen in the attached gif. Look at the W and H numbers in the info panel and compare with the size of the selection rectangle to the left.
For larger selections where I can't judge the selection size by eye, this makes it impossible to get the size right and my selections often end up being wrong by one pixel in width and/or height.
The issue is likely caused by the info panel accessing a size in floating point numbers and rounding it to whole numbers using different logic than the actual selection rectangle uses rather than accessing the same already calculated whole-number values both places.
For any pixel-perfect selection work, this bug is critical.