I understand if I use Automatic/Fit for settings in ImageSize, WindowSize, etc…, then when one selects a magnification setting the created window/palette will enlarge correctly. This is great for us who are visually impaired. However, when I use numerical values for ImageSize, WindowSize etc…(for elegant design of layout) then the zoom enlargement gets screwed up. I am seeking a method to do the scaling programmatically so I can do the scaling correctly ie some kind of factor to be applied when the user selects a zoom setting and not let the FrontEnd do it for me. This solution would help many UI designers using Mathematica with full visual control. Consider this programming challenge for greater accessibility. Sorry I have no clue where to start so have no code to offer. The trick is manually handling the enlargement not the FrontEnd for complete control. Thanks for helping us visually impaired folks. 
Attached is an example where the “fixed” window size follows magnification.
I have also tried to use NotebookDynamicExpression to directly connect window size with the magnification popup. But without success.
Meit zoom.nb (10.5 KB)
Thank you! Will explore your nb.
Updated the notebook
Meit zoom 2.nb (9.9 KB)
Third version, which fixes a nuisance. Changing magnification with the popup at lower right corner and then pressing palette -/+ buttons could cause error messages:
This does not happen if AbsoluteCurrentValue[..] is used instead of CurrentValue[..]
Meit zoom 3.nb (4.63 KB)