The issue you’re seeing might be associated with the changes described in this wolfram technology conference presentation.

Check this: https://mathematica.stackexchange.com/questions/216602/how-to-increase-the-font-size-in-context-sensitive-autocompletion

You are right. There is no difference in Option Inspector.Formatting Options.Font Options.

But nevertheless, the appearance is totally different. The following picture shows a notebook Text-cell and an Input-Cell. Once opened with the new 12.1 desktop (Wolfram|One). Once with an elder 12.0 desktop (Programming Lab).

The 12.0 version in the foreground is what I expect. The 12.1 version in the background has far too large fonts. Despite the fact that font sizes are 15pt (Text) and 13pt (Input) in both cases.

This happens on a Windows 10 laptop with a 1920 x 1080 pixel display.

Unfortunately, from my experience, this is an important area where the documentation is uncharacteristically poor. And I haven't seen any mention in the 'release notes'. For my work I will need to understand the implications of these 12.1 changes, but haven't had time to look at it yet and probably won't do so until I hit a snag. In the meantime I'd suggest that a WR insider would be best placed to offer some guidance. Sorry I can't be more helpful, Ian

I understand that under Windows fonts now look larger. What I don't understand is why some fonts look smaller. In particular I am talking about the suggestions when typing a command and the command templates as shown below. These are now tiny regardless of the magnification setting:

This problem goes away if I change the MS Windows 10 magnification from 125% to 100%.

Thanks, David. This Global Preference makes fonts, graphics, etc. smaller. Even a bit smaller than in the former 12.0 version. Good enough as a temporary bypass.

@Hans Milton

What product are you using and please share your release id as well by running the following command:

SystemInformation["Kernel","ReleaseID"]

In[3]:= SystemInformation["Kernel", "ReleaseID"]
Out[3]= "12.1.0.0 (6764119, 2020031402)"

My display is 1366 x 768

In[4]:= CurrentValue["ConnectedDisplays"]
Out[4]= {{"Region" -> {{0., 1366.}, {0., 738.}}, 
  "FullRegion" -> {{0., 1366.}, {0., 768.}}, 
  "PixelDimensions" -> {1366, 768}, "BitDepth" -> 32, 
  "Resolution" -> 72., "Scale" -> 1.}}

Here the complete information for my system:

I am running Wolfram|One Personal on a Lenovo ThinkPad laptop workstation W530 with a 1920x1080 Display under Windows 10 Pro in Germany.

In[218]:= SystemInformation["Kernel", "ReleaseID"]
Out[218]= "12.1.0.0 (6776260, 2020031901)"

In[219]:= SystemInformation["Devices", "ScreenInformation"]
Out[219]= {{"ScreenArea" -> {{0, 1152}, {0, 648}}, 
  "FullScreenArea" -> {{0, 1152}, {0, 648}}, "BitDepth" -> 32, 
  "Resolution" -> 120.}}

In[220]:= CurrentValue["ConnectedDisplays"]
Out[220]= {{"Region" -> {{0., 1152.}, {0., 648.}}, 
  "FullRegion" -> {{0., 1152.}, {0., 648.}}, 
  "PixelDimensions" -> {1920, 1080}, "BitDepth" -> 32, 
  "Resolution" -> 120., "Scale" -> 1.66667}}

In[221]:= Options[$FrontEnd, "Magnification"]
Out[221]= {Magnification -> 0.8}

The last Magnification setting is a bypass which was set by Options Inspector->Global Preferences->Notebook Options->Display Options->Magnification->0.80 or programmatically by:

SetOptions[$FrontEnd, "Magnification" -> 0.8]

Hi John, you say that one should uninstall the former 12.0 version before installing 12.1. Actually the standard Windows installation process does exactly that and after 12.1-install c:\Program Files\Wolfram Research\Wolfram Desktop\12.0\ has no more files anymore. Hence I cannot see what a manual uninstall could change and how it could be done. The former product Wolfram Desktop 12.0 is no longer visible within system settings and therefore cannot be uninstalled.

For your other assumption that an old stylesheet default.nb could be used: The old stylesheets are uninstalled and do not exist anymore. To be sure I changed the stylesheet to anything and then back to Default. This doesn't change anything with our font size problem.

Maybe install/uninstall works totally different on MacOS. But I run on Windows 10.

Dear All,

For your information only.

See also link description here - Hyperlink: https://community.wolfram.com/groups/-/m/t/1935623

Regards,....Jos