Apple - How to prevent Terminal from resizing when font size is changed

Old question here ... but thought I'd add that this has been supported in iTerm for almost two years now https://gitlab.com/gnachman/iterm2/issues/2900

Just uncheck the "Adjust window when changing font size" box in the preferences.

2020 Update

The box is now here:

As far as I know, you can't do this in Terminal (I don't use iTerm, so I can't speak to that). The reason is that Terminal's window sizes are defined by characters, rather than pixels. Hence, when you change fonts, the window resizes based on the proportions of that font, so as to preserve the format of the existing content on the screen.

It sounds a bit silly at first, but given that a terminal simply displays only text characters, it makes sense — if a window is sized by pixels, you'd often end up with wasted space when the size of the window doesn't exactly match a character boundary.