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.