Hello vien11,
I am Matt, an Independent Adviser and a specialist technician. I would like to assist you with this and hopefully I can help you to resolve this issue promptly.
Unfortunately windows explorer does not have font size settings built in that allow you to adjust the size of fonts inside it. You can use system scaling in display properties which you already know changes more than just explorer.
It may be possible to change this via a registry setting however that would not be advisable.
You could also look into some 3rd party software such as "Alternate Font" that allows you to customize certain areas of windows with different fonts etc. I am unsure if it will do exactly what you require though.
https://www.ghacks.net/2018/06/23/customize-fon...
I know this isn't the answer you was looking for unfortunately. Best of luck finding a solution. I personally find the windows scaling feature great and easy to turn on and off when required. I run my Surface Book at 125% mostly but sometimes up it to 150% when using some software. I know you mentioned you have tried this but for others that may read this you just...
Right click on desktop and select "Display Settings" then change the Scale setting as required. And doing this is much more advisable over 3rd party software or modifying registry or system files.
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Kind Regards,
Matt Horsley