They should appear at these sizes for the various locations when used in the forum, although in the reactions menu in ACP they appear at their actual size.
Of course you can override those default sizes in extra.less template.
I found when adding my own images, best to make them bigger than display size to look best, mine are 64px so twice the size of the largest size for best display on retina screens.
The is why I resized them to 64px before uploading.
If they images themselves are large, then you are loading unnecessarily large file sizes which is not great for page load speed and subsequent possible SEO issues.