Change default image size in attachment pages

Hi, I would like to change the default size of image in attachment page. I have searched the forum and found a resolved thread about 8 years ago. However, the solution cannot be applied in my case.

I have approached the Theme owner for help but their advice was to hire someone to customize the file media.php in wordpress. One of my attachment page here for reference.

As the theme do not have a straight forward single page, I was not able to use the solution suggested by the thread 8 years ago to create an attachment.php file. Are there any other way which I can change the default image size in attachment pages?

Thank you and look forward to any advice here.
bk