Using Share->Export->Map to export a GeoTIFF but I can't set the resolution (dpi), only the image size which is not useful to preserve the a ground pixel size. Any suggestion on how to enable the resolution setting?
Export to TIFF—ArcGIS Pro | Documentation
DPI
The resolution of the export file in dots per inch (dpi). DPI is only available when exporting a layout. When exporting a map, the dpi is calculated automatically.
which probably means you have to increase your width and height in pixels
The short answer is that there are no "inches" in this scenario - there's no page to which you're writing the image.
For your use case if you want more pixels (resulting in a higher resolution image), you can request more pixels either by using the spinners or by typing in a higher number.
If you leave the lock engaged, the aspect ratio will remain the same even if you change the number in one or the other dimension.