Check your JA Thumbnail plugin (Thumbnail for Joomla content) if you want to set it up so that you see all images on your site as wysiwyg, disable the integration mode.
If you just want to do this for specific articles, and not the whole site, you have to insert {jathumbnail off} in the article. I insert at the beginning of the article. Note: the plg_jathumbnail plugin intergration mode should be set to “auto”
If the plg_jathumbnail plugin is in integration mode “auto”, you can change the size of the image in the plugin settings. If you have disabled or used {jathumbnail off} in the article, use your editor to change the size.