I use the default in eM Client which is 12pt. I get a lot of mail from MS Outlook which is in 11pt, and even on the computer monitor the difference is noticeable.
The client can change the default viewing size for messages that do not specify it in the HTML. e.g. By default Thunderbird does not set a font size, so for messages from Thunderbird, and plain text messages, eM Client will use the font size you specified in Menu > Tools > Settings > Mail > Read > Preferred Style.
Exactly. Some clients can overwrite the font settings you use, and if the receiver is choosing to view messages as plain text, it does not matter what font size you use. On my phone I view messages as plain text, and the settings I have in that app determine what size the font is.
But as I said, on my computer, I notice the difference between 12 and 11pt. Messages from this forum have 12pt body, which I think that is pretty much the standard.