Optimal Image Size for uploading, Post Thumbnail Settings

We have OceanWP theme:

1) What is the optimal image size (width x height) for image uploads to the media library?

2) If we want our images to resize correctly, do we need to upload the same image in every size we may potentially need?

3) Where can we “customize” our post thumbnails so they all appear 300 wide x 300 height when listed out on the archives/index page. We just need all of the featured images to be symmetrical.

We have tried setting these dimensions in custom image, but the theme does not respond. Can anyone explain why this is not working please?