With respect to what you tried using set-face-attribute:
The error message tells you that the first argument needs to be a symbol. At that point you should use C-h f set-face-attribute, which tells you that the first argument needs to be a face.
Here's an example of its use.
(set-face-attribute 'default nil ':font "American Typewriter" ':height 150)
Try buffer-face-set, which accepts either a face name like (buffer-face-set "error") or a property list of face attributes and values like (buffer-face-set :weight 'bold :height 150).
I think your specific example would be something like (buffer-face-set :font-family "American Typewriter" :height 150).
You can M-: and at the "Eval:" prompt enter one of ...
In Emacs the appearance of certain text is controlled using something called "faces". You should learn about faces and how to change them by reading the documentation.
To quickly change how something looks you can often put point (aka the cursor) on that thing that you want to change and then type M-x customize-face. That will often offer you the ...