I added the yahoo mail app to the start menu in my new "edge" browser after upgrading to Windows 10. It easily connects to my yahoo mail inbox page, but the font is small and difficult to read. I can press the ctrl button and tap the + button to increase the size, but I must do this each time I view my yahoo mail page. None of the "settings" adjustments in yahoo or windows 10 seem to make a difference. The Microsoft tech support guy told me this was a yahoo issue and not Microsoft's problem. I cannot find any answers in the yahoo blogs and yahoo doesn't seem to have any interactive tech support. Does anyone have a solution? or a way to interact with yahoo? I did find some 3rd party tech support that said they were "yahoo," but were actually trying to sell me some bogus malware protection. 

Hi,

Do you mean the Yahoo mail page as below? It seems a known issue.

http://yourbusiness.azcentral.com/yahoo-mail-font-small-16561.html

And the workaround should be ctrl+ mouse scroll.