The default size for all images uploaded in wordpress are –
- 150 X 150 thumbnail,
- 300 X 300 medium and
- 1024 X 1024 maximum.
When constructing a new site, I usually change these to suit the page width for each website theme I’ve used. For example, the maximum embed size on this page is 750 pixels.
To change these sizes to the ones which are appropriate for your site, simply log into your website dashboard and navigate to SETTINGS >> MEDIA. See image below.