Hello @gigigaskell,
The theme has already provided the option where you can manage the image size on the archive page.
For this go to Admin Panel > Appearance > Customize > Theme Options > Layout Options > Image in Archive option and chose the option “Full(original)”.
Similarly, you can set the image size for the single page as well from here.
Further, please regenerate all the thumbnail using the plugin given below:
https://wordpress.org/plugins/regenerate-thumbnails/
Hope this helps.
Best Regards!!
Have a good day 🙂 .