When I use the standard WordPress gallery, I get very small thumbnails in the content:
Since there is lots of space in the content, I would like to display bigger images. How can I do that? There is no specific gallery code added in the theme.
Either use one of the other built-in image sizes, for instance
or "large" or "full",
and/or customize the sizes for "thumbnail", "medium", "large" under
or add a new image size and use it