Hi Selina,
You can try using Regenerate Thumbnails plugin to view all registered image sizes on your site.
https://wordpress.org/plugins/regenerate-thumbnails/
Once installed, you can go to WP Admin Dashboard > Tools > Regenerate Thumbnails and you’ll be shown a list of all registered image sizes on your site.
As for actually changing the sizes, for the default ones, you can change on WP Admin Dashboard > Settings > Media. For the other registered ones, you’ll have to change them through whatever plugins added them or by PHP snippets.
A wise man once said:
"Have you cleared your cache?"