Q: photo dimension reduction

Are you dragging the photo from the Browser to a folder?  To preserve the full pixel size use the command "File > Export". if you export an edited version and not the original image file, use an Export preset that preserves the pixel size, for example  -  JPEG Original Size.

If the pixel size changes even with the export preset "JPEG - Original Size", check if this preset is still correct by setting the Export Preset to "Edit".

In the Edit panel check the setting for the size "Size To":

?  ?   ?  ?

The second screenshot is from a completely different discussion.  What should be shown here is the Edit panel:

Select "JPEG - Original Size in the left pane of the "Edit" panel, then check the setting for "Size to".