Is there a basic way to resize featured image? The default size is too big!

Just figured it out myself a few minutes after posting this question.
All I did was resize the image prior to uploading it to wordpress.

I realised the image is now a bit blurry..

Can anyone help? Can you change the default featured image size in the PHP?

https://www.ads-software.com/plugins/simple-image-sizes/ Lets you set your size.

Tried the simple Image Sizes plugin, but no matter what settings I change the post thumbnail too, the existing post thumbnail image stays the same size, even after regeneration. Newly uploaded images still stay large. It’s like another setting is overriding it. :/