Something is overriding my emacs font size - emacs

Question

In my .emacs file, I have (set-face-attribute 'default nil :height 200) as the last line.
When I start up emacs, I actually see the text flash briefly to this size, but then quickly get resized to the default size (height 110).
How can I discover what's overriding my font setting?
My OS is Raspbian GNU/Linux 10 (buster).
Update: I've tried removing everything from my .emacs file, and then using M-x customize-face default to make the font height 200, as suggested by #Drew. My .emacs file now looks like
(custom-set-variables
;; custom-set-variables was added by Custom.
;; If you edit it by hand, you could mess it up, so be careful.
;; Your init file should contain only one such instance.
;; If there is more than one, they won't work right.
)
(custom-set-faces
;; custom-set-faces was added by Custom.
;; If you edit it by hand, you could mess it up, so be careful.
;; Your init file should contain only one such instance.
;; If there is more than one, they won't work right.
'(default ((t (:inherit nil :stipple nil :background "white" :foreground "black" :inverse-video nil :box nil :strike-through nil :overline nil :underline nil :slant normal :weight normal :height 200 :width normal :foundry "1ASC" :family "Liberation Mono")))))
However, I'm still seeing the same effect on startup. A brief flash of size 200 and the font getting resized by something after that.

Answer 1

set-face-attribute with nil for the frame sets a face attribute for the frame that's selected when that function is invoked. Don't use it in your init file unless you know which frame will be selected when it's invoked, and you want to change the face on that frame.
Either customize face default or customize default-frame-alist (its font, color, etc. parameters). I suggest M-x customize-face default. That affects face default in general - the default for all new frames.
Beyond that, it's possible that something in your init file is changing the font of the default face later on. When you do M-x customize-face default you can see what the latest value of the font attribute is.
If you think something in your init file is messing things up, bisect your init file to find it. You can use command comment-region to comment out a block of code or (with C-u) uncomment a block of code. Comment out 1/2 of your init file, then 3/4, 7/8, 15/16,... till you find the culprit.