I don't think you can do this with multiple objects selected, but there are ways to do it faster manually:
The inputs for the width and height in the toolbar while in select-mode might be a quicker way to change the size as they update when you change selection. Also, lock the aspect ratio (the lock button between width and height inputs), so you only have to change one dimension each time.
Pretty much all numerical input boxes in Inkscape allow entering mathematical expressions (e.g. '123+456' or '123*4') and it will calculate it for you. You could simply append '-1' to a value, press enter, and it will reduce the number it currently has by one.
Lastly, pressing Tab cycles the selection through all objects in the current layer. So you change one, press Tab and change the next selected object.