Problems with Font Size under Mac OS Ventura

I work with a 5k, 40 inch, LG monitor. The size of the fonts is very small. I can handle this. However, what me really puzzles is the following: I adjusted the font size on Outlook and on emClient to Arial 12 pt. In e-mails sent with emClient, the letters are much smaller than on Outlook (old version for Mac) although the same font (Arial 12 pt.) is used. This occurs on the same monitor. How is that possible that the same font size (Arial 12 pt.) is shown entirely differently with emClient on the one hand and Outlook on the other hand? What is the solution? Shall I simply adjust the font size on emClient to about 20 pt.? If so, is there any risk that certain recipients will see the font as really 20 pt. (i.e. much too large)?

Best, Thomas